I think you'll be fine getting him in free. Personally I'd buy the tickets online that way you won't have to stand in long queues and can go straight to the turnstiles, where you show your tickets and they just wave you through. If he's in a pushchair I doubt they'll check.
Then once in there I'd get him measured on a ride and see if they'll let him on (with an adult) slightly deceitful yes but every penny counts these days and he'll prob only go on a couple of rides.