Unknown Malaysian Disease

My family came back from Malaysia last Friday. We were there from last Monday, to last Friday. It’s now Tuesday.

On the day we left Malaysia ,Friday, my little brother complained of bug bites on his butt. Due to his usual childishness, we ignored it.

Now it is Tuesday. He just showered and showed me his “bug bites.” They do look like mini bug bites, but they are on his chest, butt, and top thighs. I don’t know how bugs can get up there. He was not bitten anywhere else during our trip.

Our Malaysian trip contained hiking through rainforests and jungles, long bus rides, and swimming in the sea on a tropical island.

Any ideas on a possible disease or treatment would be preferred. Thank you.