Have a search for 'SM58 pop filter'; this will bring up a series of foam covers for the mic' dome. Inexpensive, but would probably need replacing quite often. If he's biting the dome, he's far too close to the mic', of course, but, if that's how he sings...
There's a big difference in performance, too, between an SM58 and a Beta 58 (the Beta is a far better mic', and the dome grill is deliberately more dent-resistant compared to the SM58...). It's not a Good Idea to replace the dome completely, as it's there to protect the capsule, so a 'soft dome' would protect his teeth, but not the mic'..! Hope this helps.