This could have been a Tick? (the bug that was stuck to his skin?)
Now when Ticks are removed, there is usually an area of inflammation left behind, which can be raised like a small bump for a day or two. This doesn't always get infected by any means, as the dog's natural immune system usually copes, and the lump goes down.
If by any chance tiny bits of the Tick's mouthparts or the head of the Tick is left behind, it can sometimes cause a boil or small abscess.
If that's the case, he would need that drained, and would need some antiseptic wash on it, and possibly a course of antibiotics too. It wouldn't be too serious, just better to be treated to stop any infection spreading, and it would be best if the vet does it.
My dog has had many many Ticks removed. They always leave a little swelling behind, which goes away in a day or two. She got a small abscess too once, from a bit of the Tick which got left behind. But she was fine after treatment. She's a very fit happy girl.
By the way she never got Lyme disease from a Tick bite.
When a Tick is removed, it is always wise to bathe the area thoroughly a few times afterwards with an antiseptic, even if it is removed well and entirely.
It is also possible (though this doesn't happen as often as is commonly thought) that the Tick can carry Lyme disease or something similar which could cause the dog to have a feverish illness, and can cause arthritis afterwards. To be on the safe side when a Tick is removed it's wise to keep it in a jar for a few weeks. It doesn't matter if the Tick dies. But if the dog does get any later symptoms, the Tick can be tested to see if was carrying Lyme disease, which can help diagnosis.
When your mum took the (Tick??) off, it would have just hurt your dog a tiny tiny bit -probably felt a bit like a pin-*****. And dogs won't like that! So that's why he went running off. My bet is he'll come back, most likely when he's getting tired, fed up, or hungry. So do try to stay calm. He'll be fine, I'm sure.
Are there any friends or people living nearby that he might have run to? Check with them.
I hope he's back home very soon.