My big fluffy cat was due his booster in May but I was pregnant and feeling hellish so my Dh attempted to take him twice. The first time he popped him in the basket and like a plum left him outside whilst he got the other cat in the car - he got out and ran off. What did he do the second time? Yep the same [doughball emoticon] anyway will the vet insist that Fluffer has all the jags again or will they be okay at him just getting his booster this far over?
It is likely he will need to repeat the whole course unfortunately. We have to work in accordance with manufacturers guidelines to ensure that we have effective support from them in the rare cases where we do get things going wrong. Certainly for the brand we use they say for cats that over 3 months overdue requires a full course again (only one month for the leukaemia part). There may be a variation with other manufacturers though so you should check with your vet what they have been advised. The only way around this with any certainty otherwise would be to get immunity checked via a blood test which financially would probably be more costly than the vaccines - and if then proven the immunity is not sufficient you still need to get the vaccines done on top of that.