Could my kitten be overwheight? He's 7 months old and weighs a bit over 4kg (8.8 lb)

I adopted him and his brother when they were 3 months old. They were malnourished and had a radical diet change from random human food like (oatmeal and yoghurt) to cat food. I feed them Prima Cat Kitten wet food and kibbles. Vet said not to free feed them and told me to "cut" their food with boiled chicken to make it less fatty because they had diarrhea in the beginning and this one still has it quite often. They were neutered a month ago and their vet said that maybe I should watch his weight a bit as a side note and we never discussed it any further but now I'm worried if I'll do some damage to him if I don't start regulating his food. The vet is otherwise amazing and the best one in town but I have a feeling they're projecting a bit about the diarrhea because they told me that they have IBS and fatty food makes it worse. But does it really work like that on cats?

I've thought that you cannot over feed kittens and I want them to get used to the fact that enough food will come regularly because when they first came here, they would tear my hands and arms when I held a package of food and I had to call emergency clinic too many times because they had eaten for example mandarin or drank coffee etc :') I still have to feed my dog in the bathroom because otherwise they will eat his food (he will let them if they try, he's very beta) but they've calmed down a lot and even sometimes leave leftovers.

His brother is a bit smaller than him but they are big kittens, this guy is already way bigger than my partner's adult cats. I've seen their father and he's a large cat too. So I'm asking for an opinion, does he look overweight or is he just built large and is there a possibility for him to get health problems if he doesn't loose weight/stop gaining it?