Hmmm well I've been table top wagaming for quite some years. I 1st started out around the age of 8 so I wouldn't say your son is too young but it would need to be quite basic (The 1st games I played were very simple affairs!)

These days you are spoiled for choice with ancient wargames rules, a quick internet search will bring up many options (plus lots of figure manufactures)

From the top of my head I'd recommend either;

DBA - this is a "quick play" set of ancient rules, it is very old and was written by the wargames research group (WRG) It requires only modest initial outlay to see if you like it. Each army is made up of 12 bases of troops. Each base is typically 3/4 Infantry or 2/3 cavalry. If this does strike your interest once going WRG also produce a set of rules called DBM. The basic mechanics remain the same but are expanded (Each class of troop is further broken down into superior, ordinary, inferior or special types for example) and is designed with much larger battles in mind.

My other recommendation would be Warhammer ancient battles. Whilst some might baulk at the affiliation with Games-Workshop the rules are simple enough to learn (they are based on fantasy battle) and actually IMHO provide good enjoyable games (A major factor when playing any sort of game!)

As for clubs. Well in my time I have attended 2 clubs. 1 was very "clicky" and I never really felt welcome, 1 was very friendly and welcoming and I stayed with them for some time. So my advice would be to go along and see what its like with the added bonus is if they are of the 2nd type then they'll probably help you learn some rules and give advice on getting started.

Eek sorry about the essay - hope that was some of what you were looking for