It would have been way better for him to have been with Mum till 8-9 weeks of course ( and in no way am I saying that it is ok that the petshop is selling them this young as it isn’t ok! And I hope one day there is rules against it )
However, that’s not your fault of course and I’m glad he / she has a owner that wants to learn and do all he can for him / her from here
How long have you had him ? Is it first day or 4-5days? ( to get an idea of how long you have had to be able to assess his eating etc)
Does he seem fully weaned? Eating the amount he should?
About his body size in hay daily ?
Do you have him on a good brand of pellets? I use oxbow but there are a few other great ones too ( make sure you don’t change his pellets brand quickly though if he isn’t on a good one
leave him on it for at least a week since you just recently got him / her then change it very very very slowly – like over a month or 6weeks as their stomachs are sensitive especially if he was weaned at 3-4wk ish age )
If he is eating normally and his poop is normal then he should be ok on the weaned early front – next I would book a vet visit – so they can check him all over for mites, worms , fleas – check his teeth and everything else over
Don’t give him any vege – and with the gassy tummy I’d talk all that over with the vet if you can take him in for check up in next day or so but most important to know is stasis can be extremely dangerous – watch for him it eating or drinking or pooing – any of the above
He would need seen straight away by a vet in that case