If it was me, I'd want to get this checked out by a vet to be on the safe side. You're obviously concerned about it otherwise you wouldnt' have posted in the first place and you wouldnt' have rang the vet. If you're happy with what the vet told you on the phone then fine, but you do still seem to be a bit concerned about it. You mention he's about 6 months - has he been neutered yet? If not, that might be a good time to get the eye checked out too. I would say it can wait until you have a routine appt at the vet, but I wouldn't personally want to leave it too long before getting it checked out. It may be nothing sinister or it may be indicative of something more serious. Only a vet examination can determine that.
re insurance - it's up to the individual as to whether or not they want to take out insurance, but I do worry a bit when people say they don't want it because it is too expensive. If you can pay on the spot that's fine, or set up a savings account for vet treatment, but please don't estimate future vet costs based on what you have paid in the past. You seem to have been very lucky so far. Vet treatment can easily run into hundreds or thousands of pounds. Blood tests, x-rays, dental treatment, RTAs... all happen to cats and can cost hundreds to sort out. When one of my cats died from a thrombosis 3 years ago, the vet treatment came to nearly £2000 and he was only at the vets for a few hours. Not taking him to the vet was not an option as he was lying on the floor unable to move. One of my boys swallowed some string from a cat toy last year and had to have an emergency operaton - cost of £450. If you can pay that kind of money on the spot, that's great. If you know you will be able to get your hands on that sort of money in an emergency (eg borrowing from family, putting it on a credit card) then that's fine too. But it's important to think ahead and be prepared for what may happen. As long as you're happy that you have thought about it and have worked out a way of paying for treatment, or have accepted that you may need to refuse treatment on grounds of cost (and maybe have a cat pts without attempting treatment) and can live with the decision you've made, then no-one can do more than that. It just worries me when people (talking generally now, not specificially about the OP here) think they don't need insurance because they think that their cat will never get ill and never need much vet treatment.