Spot on treatments are available for small animals such as hamsters etc, did you buy the correct one specifically stating mice, hamsters etc?
There are many things that it could be, if you have other animals then he could have fleas, possibly may have ringworm or scabies- these are zoonotic so can be passed on to you. The best thing to do would be to consult your vet and ensure he gets the correct treatment. In the meantime dont use the treatment you bought again, unless you see a flea.
Have a good look all over him, and see what u can find this will really help the vet when you see him/her.