Is 39lbs good for a 4 year old boy?

Roman is 4 years old and 39lbs and i think that is about right for his age but every time his grandpa (fiance's dad) sees him he always accuses me of not feeding him and being a bum cause i never seem to be at work when he comes over ( im a fitness trainer so i work from 5-10)...I asked one of my co-workers (nutritionist) but he had no idea cause he specialized in more fitness/bodybuilding but he says that he doesnt look to skinny/fat...