My brother has two huge bug bites on his arm, help?!

My 14 year old brother has two very large swollen bites on his left arm, about 2 inches in diameter. He got bitten in the same area 2 weeks ago, and it went down after taking some antibiotics. Now there's two more bites in the same spot. He doesn't have any symptoms other than itching. We would just like to know what is biting him and if we can prevent it.... I know he's sleeping on this very old foam matress so it could be something living inside it. yuck. anyways any help would be appreciated!