A lot of clubs have a 10 year old rule in place,

However id imagine some are more flexable with their junior classes than others.

For insurance reasons theres no way your kid will be put in a ring, and again i imagine a lot would depend on his attitude.

At only 5 he would have to be quite disiplined, take instructions, do whats asked without complaining or shouting or running about disterbing other kids.

The other thing of course is you want him training somewhere you are happy with, im not sure if he intend to watch every session, but boxing 'classes' are not like other martial arts where they take place in the local lesiure centre with the guy who runs kids parties as well. What i mean is the mentallity even at junior level is not the same as 'classes' for other martial arts. Thus you need to a find a place where the trainers do have a certain amount of compassion for a younger kid.

As for finding a gym, its just a question of ringing up your local gyms and seeing whcih do kids sessions and which are happy to give your lad a try.

Just dont end up making the boy do boxercise! :P