That looks very sore poor little mite
Did any pus come out of the wound? It is possible that it was caused by a fight and after a period of separation I think you may not be able to put them back together though as they will lose the scent bond.
How old is he? If is does not heal it could be a tumour. Because it looks deep I imagine it will be affecting the eye and the tear duct too.
I hope he gets better soon. You need to give the meds by mouth and not in the water just in case they said to put in inthe water. Hammies do not get the right dose in water and sometimes do not drink it.