When should I take my pet rabbit to a Vet

I have a year old Lionhead who I took to the vet last year. He's healthy and happy but I'd just like to know. Do they need a check up ever once in a while? Or do we only take them in when they are acting different. He's happy and healthy

Personal Preference. Some people will suggest never unless their sick, some will say every year, some every 6 months.

IMO:
Ideally once they get older, say 5 for small/med breeds, perhaps younger for giants, it wouldn't hurt to bring them in for annual checkups and bloodwork to make sure everything you can't see is functioning properly. Having their teeth checked regularly wouldn't hurt either but that should be done at any age. Catch something before it happens, not after.

I agree with Watermelons. I would say in an ideal world, and this is an opinion, I would say once per year for just a check up really cannot hurt. If you can afford it/have the availability, blood work every 1-2 years is also a great idea! Of course, doing both more frequently for the older animals is also probably a good idea. Even if you know a lot about rabbits (or any animal), having someone else see your animal might be helpful, simply because the vet does not see your specific pet every day.

I took him last time this time of year. Would you say I should take him right now? & book it as a wellness exam?

Again, this is an opinion, everyone is different, but I'd say yes. Might as well just book a wellness exam, just to make sure everything is 100% with your little bun. I tend to be on the overprotective side when it comes to health with my rabbits, especially since they are good at hiding illness. However, I wouldn't say to need to do it immediately or anything. If it would be easier for you to do, say, in a month, that's definitely ok too.

Our rabbit savvy vet recommends once a year to check their weight, address any potential issues (like dental issues which can't be detected until they start having problems unless the vet looks in the back), and it gives me peace of mind to do this. We do have older bunnies though, and they have been to the vet more often like every six months due to minor issues. I would recommend it myself because this is how we found out that one of our buns needed a dental trim and he had no symptoms, but apparently he was in a mild amount of pain because afterwards, he was like a new bunny!

I was wondering shouldn't rabbits be taken to the vet for inoculation at least every year anyway?

Rabbits do not get vaccinated in North America. So yes if you live in an area where you have RHD or Myxo, please get your rabbits vaccinated on time!