1. At the outset, your son can apply for a divorce where he is ordinarily resident or where he was married, if that is a different country. However, if he can choose which jurisdiction where he could be resident to get a divorce, the getting a divorce is much easier in the UK , as your son and his Mexican wife only need to be two years living apart to get a divorce in the UK if they both agree to this course of action as you seem to suggest in your query by stating they both want a divorce. In Ireland, the parties have to be four years living apart in order to be granted a divorce. So there is a much longer wait time before getting a divorce in Ireland than in the UK. The divorce law in Ireland and the UK are broadly similar but with the caveat that the woman is better provided for under the Irish divorce system with greater payments being received by wives in Ireland.
Obviously, UK would be better then, will he need to engage a UK solicitor to initiate divorce proceedings?
2. Yes, it would be better to hire a solicitor. Whilst, in theory, your son can do his own divorce, it is much easier if he has someone to guide him, especially as his wife is in Mexico.