STICK TO YOUR GUNS!!!
Your son is fine. My daughter is about 35.5 inches tall, 2.5 years old and weighs about 27lbs. She too is very active and we limit her sugar intake. She eats VERY well. She is not a picky eater.
I would say, if your doctor isn't concerned and your son is getting good balanced nutrition then tell the rest of them to back off! Refined sugar is not healthy. On occasion, as a treat, is fine. It teaches them to have a good relationship with food. That is my primary goal. To teach my daughter to eat when she is hungry and eat foods that will sustain her, rather then eating when she is bored or whatever.
Probably one thing you are battling against is the fact that childhood obesity is on the rise. More kids then ever are overweight... so everywhere you look you see these chunky kids!!! When you have a NORMAL, healthy child... they look scrawny... when in fact, they are HEALTHY!
I would have a serious talk with your husband and let him know it is NOT ok that your in laws give your son all this junk food. Why teach him habits he will only have to unlearn later when he is older.
There is tons of information out there on childhood obesity and diabetes and such. Do your research. Talk to your pediatrician... let him know what you are battling against and he/she can probably give you some good information. Then, go to your husband with the information and in a calm manner have a discussion.
Of COURSE you control what your son eats!! He's only a toddler! If we didn't control what they eat then they WOULD eat just junk all the time. They don't come out of the womb knowing how to make good choices. We have to teach them how to make those good choices.... and part of that is teaching them how to make good food choices. That's what parenting is all about. It is not about giving into your child's every whim.
Good luck. You have your sons best interest at heart and I know you want him to be healthy. You are right to limit the junk food intake. You are right to want your son to make healthy food choices. You're being a good mom.