15 months is better than 9 months which is what used to be advised. When it was the norm to forward face at 9 months, if you looked at child accident deaths across Europe, there was a spike in number of deaths at around 1 year old. This is because younger children were in rear facing seats, which are much more protective, and older children are more able to withstand the impact of a car accident even though they are forward facing. This is why the newer car seats have a ban on forward facing younger than 15 months, but it does continue to have a benefit for children 1-4 years (and beyond) - it's just that when you look at large numbers statistically it is harder to see because there aren't many children still rear facing at these ages outside of the Nordic countries, where it has always been the norm simply because when they first started making car seats (before Europe had a centralised safety standard so every country made their own) their first ones happened to be rear facing, the designer was apparently inspired by rear-facing astronaut seats. Incidentally if you do compare with Sweden, they have very good accident death statistics for children, the lowest in the world. Their accident deaths for children 1-4 years are lower than across the rest of Europe where it is the norm to forward face after about a year still.
Ultimately it's your choice, but personally I would keep rear facing at least until either your child is uncomfortable, you need to move them for another child (and you decide against a larger rear facing seat) or you max out the limit of your seat. I have the Joie 360 spin, and it's not the most generous for leg room, there are certainly rear facing seats which have more, but little children don't mind at all. In fact it is often more comfortable for them to have something to rest their legs on rather than have them dangling. He might surprise you! Just keep going and see how long it works for. I wouldn't rush to forward face as soon as you can. We put a dangly toy on the car headrest and had a mirror so he wasn't bored or lonely in the car.
With DS1 I used a forward facing seat once he was out of the baby seat but I kept him in it as long as possible - 18 months. This was 2009 so ERF seats weren't easily available then and I didn't have my own car. The only isofix one was about £400 and most cars didn't have isofix anyway! There weren't any belt fitted ones without tethers back then.
With DS2 we had the Joie 360 Spin and I had planned to keep him RF until 3 or 4 but DH really wanted to turn him around 2 so I agreed to do so on occasions he was really unsettled, for shorter slower journeys, and we'd RF if he was likely to sleep or for faster/country roads but DH took this to mean any time he felt like it, and it happened to coincide with him taking him in the car a lot without me so he started to prefer FF and was FF basically all the time from 2.
DS3 is 16mo and is still rear facing, I would like to keep him RF through all next winter (so until about 2.5) as the first winter DS2 was FF made me really anxious in the car. But I don't feel too anxious about turning him from 2.5 - 3ish. There is still a benefit to RF at those ages but I didn't feel it was as critical as when younger.