Unchanging dark-colored bumps probably are not warts. Depending in part on his age, I would wonder about
molesBirthmarks - pigmented
Gestational trophoblastic disease
Hydatidiform mole
Pth or a condition called seborrheic keratosis. These are not sexually acquired or transmitted. And if they are warts, it is definitely not proof of infidelity. To understand why, read the HPV information in the link at the top of this forum ("STD Quick Facts and Articles") as well numerous other threads about HPV infection and transmission. No, there is no test to know whether you are carrying HPV. But you may be; almost all sexually active persons get infected with HPV at one time or another. (See the same article and threads.)
Bottom line: Your husband is being irresponsible in not seeing a doctor for a diagnosis, if only to give you peace of mind. But nothing can be said from this information about his sexual fidelity.
Good luck-- HHH, MD