The first step would be to determine if his employer sponsors a 401(k) plan. You can determine that with a general internet search. This won't tell you if he participates, but should at least tell you if the company offers a plan. You may want to do some research into a Qualified Domestic Relations Order, or QDRO, which is a clause or part of a divorce decree which gives one spouse certain rights to the other spouse's retirement benefits. You may also want to visit with a divorce attorney for an hour or two, who could offer guidance more specific to your situation.
Hope this helps.