Hi sorry for the late reply.
It's often unfortunately the case that these seats are sold as being from birth/all stages but you don't get a great fit for newborns in them.
If you have an infant carrier, you'll normally get a better fit in those types of seat and could switch to that temporarily until he is a bit bigger.
If you are stuck with this seat I can try to get you a better fit with it but it might not be great even with tips.
So first of all, he is sat too far up in the seat, his shoulders are above the bottom of the head support. You want to line his shoulders up with the shoulder straps. Loosen all of the straps fully and lay them over the sides of the seat, before you try to put him in, including pulling the crotch strap out of the way downwards. Then put your hands under him on either side so that you can move him down. This will bring his hips further down and in more of a lying position. It might be that you need to bring the headrest up one slot, although this would be very unusual, so I would try to fiddle with his position yourself so that his nappy is in contact with the crotch strap and his shoulders are just touching the shoulder straps and he is not lying on top of them. I think he is sitting too high up.
If the car seat is not on full recline, also make it on full recline. However, the recline of multi-stage seats is often not reclined enough for a newborn baby, especially if you have tilted seats in your car. If you can measure the angle e.g. with clinometer app on your phone when the seat is locked into rear facing position, the angle should be no steeper than 45 degrees from flat but should ideally be between 30-40 degrees from flat.
Now with him lying between the shoulder straps and the buckle you can fold the buckle up over his nappy and then take one shoulder strap at a time over each shoulder, making sure to position them so that they touch his neck. At the moment, his left shoulder is not actually contained by the strap. You want to smooth and gently jiggle the straps so that they are positioned curving over the shoulder, flat onto the chest.
Next, put the buckle tongues into the buckle to close it. Don't tighten yet. First you need to position the hip straps. Take hold of the grey webbing just above the buckle and pull upwards, this brings the straps through to be snug on the hips. Check tiny little legs aren't froggying up and getting caught in the straps as you do this.
Now you'll have loads of loose strap above the buckle, so now hold your shoulder/chest pads in place on the chest and gently pull that tightening length of strap until all the slack is gone. Check by pulling through from the hips again and keep adjusting the position of the pads as you tighten. You should do it tighter than you think, it won't hurt him (think about how newborns like to be swaddled).
It's tight enough when you can get just one finger between the strap and the baby but not two fingers stacked on top of each other. It is important for safety for your car seat straps to be this tight.
This should be better. However, if you move the baby down and the distance from their nappy to shoulder is shorter than the distance between the crotch strap and shoulder straps, the seat might be too big. It's sometimes unavoidable to have a gap with a newborn, but more than a 2cm gap is when the seat is considered too small. Ideally you don't want any gap at all, as with rear facing the shoulder straps should be on the shoulder or very slightly below, but there is a little bit of tolerance for newborns (but you still might get a better fit in an infant carrier).
The other incompatible issue would be the angle as stated before. The closer it is to 45 degrees the shorter you'll want to keep the time in the car at least until LO is starting to sit supported/hold their own head up.
Do you have the top tether attached to the seat? I can't see it in the photo (but it might just be dark). That won't affect the baby's fit but it is essential for safety.
Attached is an example good fit of a newborn in a multi stage car seat.