After this stage you basically have three (four) choices:
Extended rear facing seat up to 25kg+ - as HiCandles says for example Axkid Minikid. This is the safest choice and if you are happy RF, then it's a really great one as you'll be able to hand down to your younger child in time as well. They take up less space than the 360 or the Stages, the belt isn't in the way, basically an all around upgrade if you're used to other types of RF seat. However due to a shift in the legislation they are a bit pricey at the moment and the older, cheaper models are hard to find. If you have time to keep an eye out for sales that's always a good thing to do.
Axkid Minikid/Move/One
Besafe Stretch/izi Plus
Britax Max Way/Multi-Tech (Apparently a new Britax one coming soon).
123 type seat - these seats are forward facing with a harness which you remove at the point the child hits the limit (which is usually 18kg/105cm but some go up to 25kg). Then it turns into a high backed booster seat. These are a good choice if you're already FF / want to FF but you don't need the harness for a long time. You need top tether points in your car to fit the majority of them. However they can be cheaper than an ERF seat.
Britax Advansafix/Evolvafix
Recaro Tian
Joie Bold
Silver Cross Balance
High backed booster seat - the final stage in car seats. Forward facing, just uses the car seatbelt to restrain the child. I wouldn't suggest this for you because at 100cm and 15kg your DS is only just barely at the minimum and it's better to wait until they are closer to 105cm and/or 18kg. But this is what you'd get later e.g. after the ERF seat.
In order to continue RF now and FF eventually when you're ready, I think you'd be best off going for an ERF 25kg+ seat and then buying a separate HBB later on. Don't try to do both in the same seat, as it's generally a compromise on safety.
Although there are seats out there which are "all stages" and do from birth to 18kg rear facing then turn into a booster, most of these are bulky and less well rated for safety. However I'll mention two as a kind of "bonus mention" - Joie Every Stage R129 and Graco Slimfit R129. These are RF up to 22kg/105cm, have no forward facing harness stage, convert to high backed booster seats. There's no independent safety testing on them yet, but having passed the R129 testing is a good sign. These are similar in design to the Joie Stages with the belt in the way and the reclined position. They're worth looking at simply because the Graco in particular is very budget friendly, perhaps for second cars etc.