Bless him - you've certainly got your money's worth out of that seat!
If it is only his legs that are coming off the end now, then he's OK to stay in that seat safety wise. It's only once his head starts to come over the top that he will be too tall, unless he is at the height limit - which is 75cm for some of the baby seats, but I'm guessing from his clothing size, he's closer to 70cm still. However I expect he would like to sit up a bit higher and more upright, perhaps?
Due to his size I'd probably look for a next stage seat from 3 months, rather than the ones from 9 months - he will likely be too small for them.
Do you have a budget? What do you want long term?
In terms of safety, the best thing to do is to keep him rear facing, luckily there are plenty of options which will support this. In fact he won't even be able to legally forward face until he is 76cm in most seats (which is no bad thing). You want to keep him rear facing as long as possible. Looking forward, it will take him a while to meet the minimum requirements for the high back booster seat stage (100cm and 15kg) - so you might want to factor in his comfort when he is age 4+ and still not able to transfer into a booster seat yet.
Two options for this - if you opt for a Swedish type ERF seat (e.g. Axkid) then these can usually be fitted with a lot of leg room to accommodate older children, so he will be comfortable as long as he needs.
The other option would be to look at spinning RF/FF seats so that once he is older (as late as possible!) if you've got to a stage where other children his age have moved to a high back booster, at least you can let him sit forward facing. Seats "up to age 4" usually accommodate up to 105cm and 18-20kg. These also sit higher up in the car due to the spinning base, so might get him a better view out of the window, especially if you remove the rear headrest.
Personally, I would stay away from the "all stages" up to age 12 seats because he's going to need a very good fit once he does enter the high back booster stage and you don't tend to get that from the all stages seats. They also quite often have to be reclined to rear face which is a bit of a pain.
What might be best to do is take him to a shop where they have different brands of car seat and try him out in a few different models to see where he gets the best fit, with which combination of inserts. He is in an awkward in-between stage when it comes to inserts, so some seats won't get a great fit for him and it would be best not to buy blind.
If I was showing seats, then I'd probably start out with Britax, Dualfix if you like the sound of a spin seat or their ERF seats if you prefer that - mainly because they sell a separate comfort insert/seat reducer which often really helps with that in-between stage, but you might well find a different brand is fine for him. Since you want a really good, experienced salesperson who isn't going to be thrown by the combo of size and age, I would go onto Britax's stockist search and see if you can find one which is not a chain. If you only have chains, John Lewis tend to be best - see if you can make an appointment and ask for a really experienced staff member.