Don't worry, he looks old enough to be weaned (around 6-9 weeks of age) though he may still enjoy milk mixed into his food.
If you contact the breeder he/she may tell you what your piglet has previously been eating so you know what it is that he is use to. A good breeder will get a piglet use to a variety of foods and offer a sample to the new owner.
Your piglet is likely stressed from being separated from his mother. While getting another piglet for him would be a lot of money, it may encourage more activity and a happier piglet. New piglets tend to be shy, so lots of cuddle time will help bring him out of his shell. They love to spend time with people and young piglets can be frightened if spending a lot of time alone. Maybe a radio or quiet TV would help make him feel safer when you cannot be there.
If you can contact the breeder they may also tell you what the piglet was use to using the bathroom in. Perhaps it was learning to go outdoors or in a litter box and this may be confusing it. You can train your piglet to go outside or to use a cat litter box with the front cut down so they can easily get in. Avoid cedar chips or pellets, as this can be toxic, but puppy pads, pine bedding or newspaper can be used instead.
A young piglet only needs to eat 1/4 a cup of pig feed twice a day (so half a cup every day), once in the morning and once in the evening. You can also break this up into three meals a day. Piglets can more easily eat small food so make sure his diet is a piglet pellet instead of that made for adults.
It is suggested a safe multivitamin with iron in it be offered to piglets since living in doors they can't get naturally occuring iron in the soil. Since neutering/spaying an adult pig is difficult, other health care suggestions include getting them neutered or spayed and getting them vaccinated for tetanus toxoid, rabies and erysipelas. Worming is also suggested for them (twice a year).
Mini pigs can get constipated so offering a little olive or mineral oil in their food or a little canned pumpkin can help get things moving.
By the way, your piglet is VERY ADORABLE!