There are a ton of kids of that age and younger who go to comic con and love meeting all the superheroes, princesses, etc etc. You'll probably want to stick with just going for the one day, and the Sunday will be a bit less crowded than the others. Don't miss out on the kids area which sometimes has little shows, story times, sing-a-longs, etc and is great for giving them a bit of a rest.

Of the two, I would recommend Birmingham as a bit better for a first time - it's a bit smaller and less crowded, plus the NEC is a great venue because as well as the convention halls there's a lot of atrium space where you can chill out and be away from the worst of the crowds.

Also look up Megacon Live - that's similar to MCM, there's a London one in January and a Birmingham one in March, and they have a kids inflatable play area (big slides, bumper cars etc) which he'll probably love.