Could be because of the female or something is off in the cage (double check the heating, did it get too hot?). And it IS that time of year for pairing up boas, so your female may be throwing some pheromones that are driving him crazy.
If everything on the husbandry front is good, you may want to try covering most or all of the front of the cage (drape a sheet over it or tape up some paper) and see if that calms him down from rubbing?
Some boas love to rub and usually a change of caging may solve it or removing his ability to see 'out' may solve it. He should be fine in that size cage.

Depending on how bad it gets, it can cause infections if he rubs raw, one of mine was in a tub and rubbed so bad in high humidity it caused the infection to get into this tooth and that required vet surgery. Moved him to drawers style freedom breeder and he was fine. I had another in a glass cage with screen top and no matter how perfect it was, he constantly rubbed the screen, so moved him to a plexi cage and he was fine (although he does start rubbing around this time of year again grrr).
A lot of times if they rub badly, the bumpy nose look will never go away (call it a 'rhino' nose).