Bumps near his hindquarters are not normal. I would recommend getting him to the vet to get them checked out. Skinny pigs are prone to a variety of problems including a variety of cancers, so I would make sure to get him to the vet ASAP whenever you suspect there to be a problem. If he's living with a guinea pig, these may also be "war wounds" if they're in a cage which is too small. Although guinea pigs are highly social and should be kept in groups, in too small of a territory they will become territorial. Most people recommend keeping a skinny pig with a haired pig because they often have a difficult time regulating temperatures and can snuggle with their friend to maintain warmth.
The scabs also sound suspiciously like mites?
We are as gods to the beasts of the fields. We order the time o' their birth and the time o' their death. Between times, we ha' a duty. - Terry Pratchett.
"Men have forgotten this truth", said the fox, "But you must not forget it. You become responsible, forever, for what you have tamed. - Antoine de Saint-Exupéry