OK I have caught up on the posts on the thread.
The photos of him in his Maxi Cosi - this is a PERFECT fit. Please keep him in here. He looks great in this seat. He will be fine in there probably until 8-9 months minimum if not all the way up to 12 months. He has plenty of growing room, and they do not keep putting on cm at the rate they do from birth, otherwise they would be about 5 metres tall by adulthood.
To add my take on some things that have been said by others:
All Maxi Cosi seats fit until 15 months
This is not true any more, this is a smaller seat with a smaller limit, plus size varies by baby anyway. However I think OP will probably get to 9-12 months in it. I think posters are either thinking of the old previous model Cabriofix (not i-size) which was really roomy and great - the modern Maxi Cosis are smaller, even the ones with bigger limits, or they are looking at the blurb on the website which is sadly not very accurate.
Axkid Move straps need to be physically unthreaded and moved down
Again this applies to the older model Axkid Move, not this one OP has which is Axkid Movekid and is a newer model. With this model, the headrest slides up and down to move the straps. In the showroom picture, the headrest is much too high, in OP's later pictures, the straps are actually sited correctly but I think this baby would be better off waiting until he is sitting independently if he can get there before reaching 75cm.
Axkids don't fit until over 12 months
The old (R44) Axkids did not, but the newer ones (R129) are from 61cm or approx 6/7 months. These are different seats even though they have very similar names. They are designed differently to meet the newer regulation.
Keeping car seats in the loft will make them mouldy/unsafe
Not necessarily - I don't know where this comes from? It seems to be accepted wisdom in all the car seat groups, but there is nothing about lofts which is inherently dangerous to car seats. You should protect it from moisture if you are going to store it for a while. So if you do tend to find things go mouldy in your loft, then it's not a good place to store it. But if your loft is dry and you store other things there with no issues, then it's fine. Or you could keep it in a wardrobe, a garage, a corner of a room, whatever storage space you have. It's probably only 2-4 months you'll need to store it anyway.
Avionaut Sky is better for tall kids/more compact/36kg limit is excessive on Axkid
It has a much shorter backrest, and the height limit is the same so no advantage for taller kids. It does fit younger children better, but this is probably not necessary, since OP still has an infant carrier. Although some seats will be much bigger or smaller than others, the amount of front to back space they take up varies between cars anyway due to the angle of the vehicle seats. In any case, as your child gets bigger, they will need less recline on the Axkid so it will take up less space anyway, whereas the Avionaut Sky only has one recline position. You might also look at whether you are fitting it with extra leg room, which won't be required at this age.
Agreed the 36kg limit is superfluous. I do agree with Margaret at Rear facing Toddlers though, she says essentially the point of it having such a high limit is that you can totally ignore the weight limit, and just look at the height limit/your child's fit.