I'm sorry but as you admit, he loves fattening food - he needs to stop eating so much of it!!
Some people are able to eat whatever they like and remain skinny forever - not me I hasten to add - but most people aren't.
You son is only 13 and his chubbiness could be down to puberty but whatever it is down to, basically he needs to eat a balanced diet and cut down (I won't say cut out, even I can't do that) on crisps, chocolate, McDonalds type burgers, chinese takeaways etc. Just the usual 5 portion of fruit and/or veg, cut down on fat etc etc.
Don't serve up chips with every meal (I'm not for one second suggesting that you do) and try and make meals interesting and different.
I follow Slimming World diet and am really good during the week, Friday and Saturday I have a couple of days off and eat pretty much what I like - if I want a kebab and chips, I have one. At least that way I don't feel deprived of anything, I can still eat takeaways, sausage rolls, chips etc. I have lost nearly 3 stone so far.
I appreciate that it is more difficult for a 13 year old who wants to eat what his mates eat, but even giving him the healthy options at home will go someway toward compensating for the cr@p he may eat at school or elsewhere.