He can be fed exactly the same as an adult piggy.
However, a tablespoon of pellets twice a day for one piggy is double the amount they should have much. Please do cut that down. Too many pellets are not healthy and can contribute to health problems.
They should only have one tablespoon per pig per day.
So now you have two piggies, if you wish to feed nuggets twice a day thats fine, but only give one tablespoon each time. That way, they will only be getting the recommended one tablespoon per pig per day.
Per pig is
Unlimited hay
approx one cup of veg per day
One tablespoon of nuggets per day
Mixed food? I'm assuming you mean a muesli. If so, then please switch to a plain pellet. Mueslis are not healthy and not recommended.
Long Term Balanced General And Special Needs Guinea Pig Diets
Pellets Or Muesli / Dry Mix?