SS8 is consistently sick after each stay at BM’s

I’m trying to keep my cool in front of DH about this but SS8 is every single week, without fail, sick with SOMETHING when he comes back from his mom’s house. I understand that kids get sick and pass things around but my BD14 and other SS13 don’t seem to have this issue (ss13 has a different bio mom). SS8’s mom has BPD and honestly with how insane she is, I wouldn’t be surprised if she’s poisoning him. In fact, that’s my first thought, and I know that sounds crazy, but if you knew this woman you’d feel the same way. It was actually slightly better when BM decided she was going to take in 3 foster kids, in addition to the three kids living in her home (so BM, her husband, and 6 kids in a 3 bdrm home…) because her attention was on the new kids, but after 6 months she got tired of the foster kids and sent them back to the state, so SS8 is back to being sick again.

I don’t even know what to do and Im worried about him - is there any type of test we can do on him at a doctor’s office to test for poison or drugs? I know how crazy that sounds but that’s where we are at with BM’s insanity.

Symptoms are always a very high fever, and stomach pain, throwing up, and him saying his “stomach is hot”. Additionally he almost always has pink eye, but I think that’s just because BM’s house is very disgusting.