Thankfully he is good, safe hands. Get him to your vet as soon as possible. Maybe he will suggest a puppy food for him to help put on a little weight. He maybe a little hesitant to wander about due to his vision problems or feeling insecure due to several rehomings.
How long has he been on the antibiotics? If he has been on them for awhile and they are not working they may need to be changed.
As for the fleas as members have said, they have found Dawn (original) works. If the poor little guy has so many fleas, he may also have tapeworm (watch for what looks like grains of rice in his bed), so try to take a stool sample to the vet with you.
Please keep us update as to how Teddy is doing.
to you for taking him