Hi MonsterMuncher, how about a Toy Box. You can fill it with little toys inside. It's something he can keep in his bedroom which may turn out to be sentimental. Another suggestion is a big teddy bear that he can hug!
Also, I know you've said you haven't seen him recently because your ex and you have fallen out. I don't know how the Law works where you live or what your situation is, but here in Australia we have a 'Family Court' system and a mother is not legally able to stop a childs father from having contact with a child. Generally speaking, there is no way a mother can refuse visitation by the father. Even if one lives inter-state. They make a semi-formal type of 'access' arrangement through this Family Court system (by 'mediation') when the parents won't agree to allow visitation rights and even before any kind of formal custody arrangement is made. You may want to consider finding out your rights if you're not aware of them, and whether you have acess to something similar as legally you should be able to give the gift to your son yourself.