It is sad that Pete cannot be released, but it is good that he has found a permanent home where he is welcome. Perhaps with time his seizures will diminish. Many seizures can be controlled with medication such as Diazepam (Valium). It also depends on what type of seizures he has: mild seizures involve a temporary disorientation, ataxia and inability to perch; more serious seizures can involve unconsciousness, vocalizing, wing flapping and paddling. There are also partial seizures, which are usually indicated by a head twitch, or a twitch of an extremity.
I think in Pete's case it is likely that some type of head trauma caused his seizures. Or perhaps heavy metal toxicity, particularly lead or zinc, although X-rays would have shown any heavy metal particles. Pesticides and organophosphates are another possible cause in peregrines, and hypoglycemia is another cause in raptors in particular.
Pete is a very handsome tiercel. I hope everything will work out well for him in his new home.
Photos of Pete from the Mountsberg Raptor Centre blog.