Hi Sela, and Welcome to you! Don't be surprised or unduly concerned if the MRI is negative and his uro still wants a biopsy, given that it would be (I assume) his first. As has been mentioned, conditions that cause a rise in PSA can coexist, and MRIs and biopsies can each miss existing cancer. The combination is one way of reducing that chance, even if the risk doesn't go to zero.
One of the unofficial mottos of the club you've joined is One step at a time
We're here to answer your questions as best we can.
Djin