Oh, I'm really sorry to hear your pup is poorly. Sending him loads of love. I hope they sort it quickly. I can't really advise on the insurance and which you should go for but someone else might have some ideas. The only thing I can suggest is to call Pet Plan and find out if all treatment regarding this period of illness will be covred regardless of the date it happens, as the illness started when he was coverd.
The only other thing I can think of is stay with PetPlan for a month or two then switch.
Hope he feels better soon.
xxxx