Where on his body is the bald spot?
He does need to see a vet - we can’t tell you what it is, but there are a multitude of reasons for bald spots in piggies from mites, fungal infections including ringworm, barbering which can be due to pain in an area. All of these things need a vet diagnosis and prescribed treatment. Without such care, the issue may well get worse (and in the case of ringworm can spread and infect all the humans and other animals in the house, it can stay in the environment for years and cause a real problem).
There is nothing you can do yourself as you don’t know what the problem is and any DIY/pet shop treatment (neither of which we recommend) could well end up being the wrong thing and/or ineffective (pet shop products generally are not strong enough).
Please take the piggy to the vet yourself. If you are not an adult, then your mother is responsible for ensuring he gets the care he needs