Hi S.~
It's wonderful you are thinking about these things and that you have utilized this wonderful website as a resource to get your questions answered. Definitely keep that up! No better place for advice than from a bunch of other moms!
As for your son's weight issue, you didn't mention his actual size or percentiles, but certainly, I would discuss your concerns with your son's pediatrician. My son was also a BIG baby. He was 9 lbs, 9 oz at birth, in the 90th or above percentiles from birth... so he started off big, and stayed big.
I think an important factor to think about is the ratio of height to weight. If he is in the 40th percentile for height (60 out of 100 children his age are taller than him), but at the 90th percentile for weight (only 10 out of 100 kids are heavier than him), then he might be a little heavy for his height. If his height/weight percentiles are similar, then it is more likely that his weight is proportionate to his height, and not so much a concern from the standpoint that his is heavy... I hope that made sense. :)
But taking that a step further, about the juice... that is something to really think about if your son is already on the bigger side. Juice does have a LOT of sugar and empty calories, and is not necessary in their diet because they should be getting all the nutrients they need from the actual fruit and veggies they eat. I did give my son juice, probably around 1 year... but I have ALWAYS diluted it 50% juice/50% water, and figured I was doing something good by diluting it... I didn't want my son to get used to the taste of full strength juice, and figured he didn't need the extra sugar. I really wasn't even thinking about the calories, but I realize now I should have been... Fast forward 2 years to my son's 3 year check-up. He is now in the 75th percentile for height, and 90th percentile for weight. Now mind you, he is still a big boy in that he looks more like typical 4 year old than a 3 year old, but he is definitely not fat... and that is not me being defensive, he is just not fat. He has long, leaner (but not skinny) legs, and a little tummy, but it's small enough that you can still see his ribs, he's that lean. Well, at age 3, peds begin looking at BMI (body mass index which compares weight to height) to evaluate the potential for kids to develop obesity, and she told me that at my son's height, his weight is too much (39 inches tall, and 39 pounds). She said ideally, he should be at about 37 pounds for his height. She said that at age 3, he should only be on skim milk (we used 2%), and only 4 oz of juice per day (we were at about 12 oz per day - three 8 oz cups per day with 4 oz of juice mixed with 4 oz of water). She said to give him water, and if he doesn't drink it, he isn't thirsty?!?! So we have cut his juice cups back to like 1-2 oz of juice mixed with the rest water, so basically, he is now drinking juice flavored water...
She said those 2 changes should be enough to gradually get his weight where it should be for his 4 year check-up. They didn't recommend a diet or anything, but naturally they suggest eliminating or reducing sugary treats, or high calorie processed foods...
The reason I gave you all those details is just to illustrate what to expect as your big boy grows, and to advise you to think twice before introducing juice to your son. For a bigger boy, it is probably a good idea to use juice VERY SPARINGLY, if at all. I wish someone had told me that!
Best of luck to you and Happy Graduation!