He was speared in the abdomen.
The Jews, therefore, that the bodies might not remain on the cross on the sabbath, since it was the preparation, (for that sabbath day was a great one,) asked of Pilate that their legs may be broken, and they taken away.
The soldiers, therefore, came, and of the first indeed they did break the legs, and of the other who was crucified with him,
and having come to Jesus, when they saw him already having been dead, they did not break his legs;
but one of the soldiers with a spear did pierce his side, and immediately there came forth blood and water;
and he who hath seen hath testified, and his testimony is true, and that one hath known that true things he speaketh, that ye also may believe.
For these things came to pass, that the Writing may be fulfilled, 'A bone of him shall not be broken;'
and again another Writing saith, 'They shall look to him whom they did pierce.'
Verse 34 (the bolded verse) did not happen after verse 33. ("did pierce" is in the aorist tense.) It explains why he was already dead and breaking His legs was not necessary.
John implies these details are really important because they fulfill two prophesies - which he quotes.
He is keeping all his bones, One of them hath not been broken.
And I will pour out on the house of David and the inhabitants of Jerusalem a spirit of grace and supplication. They will look on me, the one they have pierced, and they will mourn for him as one mourns for an only child, and grieve bitterly for him as one grieves for a firstborn son.
Edit in response to Ruminator's comment
Leviticus 17:10-12 says that sacrifices had to be bled out:
'And any man of the house of Israel, or of the sojourners, who is sojourning in your midst, who eateth any blood, I have even set My face against the person who is eating the blood, and have cut him off from the midst of his people;
for the life of the flesh is in the blood, and I have given it to you on the altar, to make atonement for your souls; for it is the blood which maketh atonement for the soul.
'Therefore I have said to the sons of Israel, No person among you doth eat blood, and the sojourner who is sojourning in your midst doth not eat blood;
John would have known about this.
Whether or not you or I believe Christ was the Messiah is not important. John says himself that he wrote his book so that readers may believe, and he was showing that Jesus was bled out because Jesus had to be bled out in order to be a proper sacrifice. If the soldiers speared Jesus after He died and blood came out of the wound, there still would have been blood in His tissues beneath that wound.
John says that when he was speared blood came out immediately (εὐθὺς) implying his heart was still beating and creating pressure to push the fluids out.
It also looks like John has a purpose in grouping those two prophecies together.
Psalms 34:20 <-- "This is not how Jesus died even though this is how the others around him died."
Zechariah 12:10 <-- "This is how he died."
In addition, something caused Jesus to yell loudly right before he died.
And Jesus having again cried with a great voice, yielded the spirit;
And Jesus having uttered a loud cry, yielded the spirit,
and having cried with a loud voice, Jesus said, 'Father, to Thy hands I commit my spirit;' and these things having said, he breathed forth the spirit.
In addition, when Jesus appears to the disciples the first time, Thomas is not with them. When Jesus appears again and Thomas is with the others, John mentions the piercing in the side again and says it is for the purpose of believing Jesus is the Christ just like he said in John 19.
And Thomas, one of the twelve, who is called Didymus, was not with them when Jesus came;
the other disciples, therefore, said to him, 'We have seen the Lord;' and he said to them, 'If I may not see in his hands the mark of the nails, and may put my finger to the mark of the nails, and may put my hand to his side, I will not believe.'
And after eight days, again were his disciples within, and Thomas with them; Jesus cometh, the doors having been shut, and he stood in the midst, and said, 'Peace to you!'
then he saith to Thomas, 'Bring thy finger hither, and see my hands, and bring thy hand, and put it to my side, and become not unbelieving, but believing.'
And Thomas answered and said to him, 'My Lord and my God;'
Jesus saith to him, 'Because thou hast seen me, Thomas, thou hast believed; happy those not having seen, and having believed.'
Many indeed, therefore, other signs also did Jesus before his disciples, that are not written in this book;
and these have been written that ye may believe that Jesus is the Christ, the Son of God, and that believing ye may have life in his name.'
It is like the hole in His side is a feature of what caused His death, just like the holes in His hands.
In Luke's account of Jesus appearing to the disciples, Luke says Jesus says He is flesh and bone - not flesh and blood.
see my hands and my feet, that I am he; handle me and see, because a spirit hath not flesh and bones, as ye see me having.'
That does not confirm Jesus bled out necessarily (it is possible He just did not mention the blood) but it is interesting Luke records Jesus describing Himself that way.
Finally, if the Romans wanted to be sure Jesus died, why not just break his legs like they did everybody else?
Fenton translation for Matthew 27:50 says:
But another taking a spear pierced His side, when blood and water came out. Jesus, however, having again called out with a loud voice, resigned His spirit.