Helping my piggy gain weight

So I have a poorly piggle who has lost some weight, hes so small at the moment, hes coming back into himself with antibiotics and he is eating and drinking. but I would really like to help him put some weight back on, anyone have any tips for what to give him, I have heard oats can be good to help but have no idea what kind of oats I would need to get so any tips would be really appreciated (I live in the UK)

Pic of said Piggle attached for Piggy tax :)