I’m sorry to hear this
He will be very stressed from the bonding and being in a new place so that will have an effect on him.
You were clearly doing what you thought best, but quarantine cannot happen for babies under four months old. Their need to constant interaction and companionship is absolutely vital so bonding needed to have been done on the day he was brought home. Obviously there is nothing that can be done about that now (it is still important to mention for anybody else who comes across this thread in the future when understanding quarantine rules for babies under four months old) but it does mean he could be a bit more stressed than a new bonded baby piggy usually is. He now needs to settle in to a new way of life.
Was he eating while you had him in quarantine or has he only stopped eating since bonding/new food?
One important step is to cover the cage with a sheet so he can feel more secure.
Have you weighed him since you have had him?
If not then definitely do so. The only way to know a piggy is eating enough hay is via the weekly (but daily when you have concerns) weight checks.
They often eat when you aren’t looking.
Put hay in all hides and in multiple other places around the cage so he doesn’t have to come out far for food.
Pellets usually needs to be changed over gradually in changing ratios over many days (ie 90% old to 10% new on day one, 80:20 on day 3 etc and so forth until they are fully on new) to allow the gut to get used to a new food. However, the issue you have is that it doesn’t sound like diet has been at all appropriate up until this point so he shouldn’t be eating his old food, your older piggy certainly cannot eat the inapppropriate pellets so you can’t put any old food in the cage for baby to eat. Luckily pellets are the part of the diet which they don’t need so if he doesn’t eat the burgess then it isn’t a problem in that respect. What is a problem is that without veg his only source of vit c will be burgess pellets.
Given he hasn’t been given any veg then he won’t know what it is and needs to learn by being shown by your older piggy. He also now cannot be given much or a very wide variety until his gut gets used to it.
You will need to introduce one very small amount of one type of veg per day so he can get used to its taste but most importantly allow his gut to adjust. New veggies or too much veg at once can cause digestive imbalances and risk diarrhoea.
I’ve added a guide below about how to introduce veg
How to Introduce New or Reintroduce Regular Fresh and Dry Food Safely - The Rules
What I would suggest is that you weigh him every morning so you can ensure he isn’t losing any weight ie if he stays the same then he is definitely eating hay.
If he loses weight (or has lost weight since you last weighed him) then you could need to step in with support feeding him a fibre rich recovery feed to keep him stable but he should learn what he should be eating by watching your older piggy.
Weight and Weight Loss Explained: BMI, Weighing, Poos and Feeding Support Levels
I hope he is ok