Some rabbits can get really angry and depressed if they have several bondmates who die. I'd definitely recommend getting one who is a similar age so he doesn't have to go through that again. There are rabbits who are content to be rebonded a day after the their friend passed away, while others can take much more time to mourn. It doesn't mean anything about how much they loved their deceased partner, instead it's based on their personality and coping skills. My rabbit (Toshi) didn't binky for several weeks after he had lost his friend and he was much more scared then he had been. This coming from a young bunny who wasn't scared of anything and whose binkies were sky high.
Before finding a new bondmate, I'd give both yourself and your current bunny time to mourn. Spend lots of time cuddling with him and feeding him vegetables. Give him time to feel sad, but if it is lasting too long or he's refusing to eat, then prioritize finding him a bondmate. You don't want him to associate a new bunny with being scared/ sad and you don't want him to withdraw.