A donut peach is essentially a flat peach, correct?
As long as he stayed away from eating the pit, he should be OK. What I've read is peaches are fine for rabbits.
That is a lot of sugar, so I would stop any treats for a bit and focus on pushing hay. Poops may be a bit squishy if he got very full on the peach and isn't eating as much hay as usual.
If it is a new food for him, eating a large quantity may cause some gastric distress (it may not too), so monitor if you notice any signs of gas pain.