This answer offers a subtle but significant adjustment to Joseph's helpful offering.
OP's main question is:
Why is the masculine pronoun "him" used to describe Israel in this passage?
The central answer to this question is that "Israel" is always "masculine, singular" in biblical Hebrew. One of the basic studies of this phenomenon is by J.J. Schmitt, who writes:1
The Hebrew Bible does not know a feminine Israel. The dictionaries consistently and coherently give the gender of Israel as masculine. In the tradition the word 'Israel' names a people. Names of peoples are masculine, while names of countries are feminine.
And, it can be added, cities are "feminine", which accounts for the depiction of Jerusalem as "Daughter Zion" (among others) in the Hebrew Bible.2 This is often a helpful "diagnostic" when reading prophetic texts, in fact: cities have feminine referents (see, e.g., Ezekiel 16 and 23 for prominent examples), while masculine referents are used for "peoples" (what we would think of as "nations").3
So, in this sense, it does not matter that "Jacob" lies behind the name "Israel", although in this passage, there is as it happens a fair degree of alteration between these two in verses 1-14, the section in which this verse (31:10) appears.
Excursus - There is one apparent exception, the phrase בְּתוּלַת יִשְׂרָאֵל (bĕtûlat yiśrāʾēl) often translated "Virgin Israel", or the like. It is found only four times in the Hebrew Bible: Amos 5:2; Jer. 18:13; 31:4, and 21. Schmitt devotes some technical discussion to this (p. 119), considering it either to be an "appositional genetive" (cf. Gesenius-Kautzsch-Cowley, §128k) in which the gender of noun and adjective are not shared, or as extending the "city" imagery present in each context. However you slice it, it remains somewhat "novel", and in no way typical.
In sum, "Israel" will typically be referred to as "he", and even in this chapter there are quite a few other examples. Where English translations offer "them", it is because they are more interested in accommodating to English style (making concessions to the "target" language) than representing the pattern present in the Hebrew text (the "source" language).
- J.J. Schmitt, "The Gender of Ancient Israel", Journal for the Study of the Old Testament 26 (1983): 115-125 (quote on p. 116). Schmitt has written a number of related articles subsequently, the one most closely connected to this question being: "Gender Correctness and Biblical Metaphors: The Case of God's Relation to Israel", Biblical Theology Bulletin 26/3 (1996): 96-106.
- This also explains (in answer to a comment on OP's question) why the feminine is not used here: that might be English usage, but it isn't the pattern in biblical Hebrew.
- For those who have access to an ESV Study Bible, the general "Introduction to the Prophetic Books" has a helpful section on this matter, "Pronouns in the Prophets". FWIW.