Thanks for your replies. He shares 60g of Science Selective pellets with his 1 year piggie friend per day. I don't top up the food when it goes. I give my adult piggies 20g of pellets per day, but I thought as he's a baby, I would put an extra 20g in for him, as I believe they are supposed to be unlimited pellets for babies.
They don't get any sugary treats, just pellets, hay and veg. They don't have sugary veg either, mainly lettuce, pepper, cucumber, with some variety.
They get daily floor time, and he spends most of the time running around, popcorning and playing chase with Charlie. He doesn't look fat, he's grown in proportion, he just looks like an adult already.