In general it is a misnomer that bisexual people need to be any more poly than the general population, or that we can't be satisfied for the rest of our life in a relationship with a person of one gender.
It sounds like the larger question could be that based on your sex drive, is that going to be enough to satisfy him in that respect. Your desire to not share him is understandable. People have to at least somewhat match though and sex drive is one of those areas where people tend to conflict a bit.
I'd definitely have the discussion with him and see what he thinks. Also keep in mind his feelings could change too, but then again all relationships should have check in times to see how it is going for each other. Poly works just as well as monogamous but both people have to want it and not offer it out of fear, in my opinion.