[souffle]

It looks like a ruptured abscess or cyst to me. Syrians don't have scent glands on the tummy like dwarf hamsters so it's not that. I think I'd get another opinion as it does look very open there and may need antibiotics. Abscesses do tend to fill up again. You usually need to keep bathing them and keeping them open and draining till all the pus had drained out and they heal from inside to the outside. He looks bright though so hopefully it will be fine.