Sorry to hear that Ollie hasnt been well.
My cat Colin was diagnosed with a heart problem when he was aged 2. It was found by my vet during a routine check-up and since he`s insured I took him to see a specialist in Kenilworth, Warks. We got an appointment very quickly, and the chap was very nice and caring.
Colin was diagnosed with cardio-myopathy (?sp), a borderline case. He had to stay there for the day, having x-rays and scans. The vet was not too worried about him especially after he jumped up onto the top shelf of his bookcase from standing! He did say Colin was not a serious case, and he did see cats with far worse heart problems. In Colin`s case he felt it was a genetic, heriditary problem rather than aquired.
In Colin`s case he felt that giving him beta-blocker meds at this stage would not make a jot of difference to him so we didnt bother. Colin is an outdoors type, and was a feral kitten from a farm. He felt that Colin would probably be fine until 8 or more years, when he may show signs such as breathing rapidly after running! The thought of medicating him daily from such an early age would probably send the both of us scatty!
Colin is now aged 5, and as yet has not had any further signs. He clears the 6` fence in a flash if he thinks I`m going to get him in (I do have him in overnight and he`s very good unless he`s distraced by mice etc). He has been hanging around the house more recently, but this is due to the bad weather and lack of things to hunt!
I also had a cat who had a serious stroke and completly lost the use of his back legs suddenly late one night. This was due to a blood clot blocking his femural artery. He had seen the vet a few months before, and nothing was noted then. He was only 7, but had had viral meningitis aged 2 which he recovered fully from. This probably set up an underlying problem for him when he was older. Nothing could have predicted either of his illnesses, and he had a good quality of life until he had the stroke.
If it was me, I would see a specialist just to get some sound advice.