It's quite a common problem.
First thing to do is check the height of the straps as others have said. In the Maxi Cosi Pearl, they are linked to the headrest. Squeeze at the top of the headrest and it should release it to slide up and down. See the pictures - you want the headrest sitting just on the shoulders so the child's head is in it but shoulders are below. The red seat shows a too-low headrest whereas the grey one is just right.
Secondly you don't want the straps so tight that they are hurting him. Pull them snug to his body but not necessarily as tight as you can go. Most people do the straps too loose, but if you've overcorrected and done them too tightly, then that will encourage escaping. If you can get your fingers flat under the straps that's OK. If you can get multiple fingers under then it's too loose. The Maxi Cosi seats have padding on the straps so be sure to position these properly too. And it's good practice to loosen straps all the way, sit child in seat, fasten straps then tighten while making sure they are in correct position.
Thirdly, don't dress him in bulky clothing in the car seat. A t-shirt, plus jumper or hoodie is OK at his age. (Hoodies are sometimes a bit too bulky for tiny little babies, but they are fine for toddlers). No coats or jackets, or padded costumes. If you've added anything to the seat for comfort, unless it's an official Maxi Cosi insert, take it out as this could be making it too loose as well.
After this you have behavioural approaches - try explaining that seatbelts keep us safe and get him to check everyone has one on. Some people do some "training" ie plan lots of fun trips in the car reminding him to keep his arms in, if he takes them out then stop the car and go home if necessary, conversely lots of praise if he's keeping arms in. Or try being as strict as you'll ever be - e.g. some people shout, even though they don't usually shout, because it's a safety issue. You can try distraction, dance moves involving his arms (if not distracting for you), another adult in the back with him, giving him something to hold - it could also be a privilege reserved for children behaving properly in the car. Reward charts - a star for every safe car trip leading to a prize - can help build positive habits and provide motivation.
There are products you can add to the seat, one type is the clip that holds the straps together as you've tried, another is a wrap around velcro panel which blocks the gap that they pull their arms through. (Called 5pointplus). It's important to check the fit before you add a product, as the last thing you want is an add on product (already non ideal) masking a poorly fitted harness. However if you've tried everything else and the product works, then it's better to have an add on product than the child totally hanging out of the seat. You can put something bulky on his wrists like a chunky silicone watch or bracelet, which can make it harder to get arms through the straps.
Last resort would be to look at changing the seat. If you can borrow one from a friend then you won't have to shell out money. Just changing to a different brand seat can sometimes be different enough that they don't try escaping and then the habit is broken after a few weeks and you can even swap back to the original seat. In other cases you have to stick with the new seat, but at least if you've tried one out then you know what you can buy. And sometimes they simply escape from every seat.
Another option would be to look at different kinds of seat. At his age you would want to keep him in some kind of harness, not a seatbelt yet. But if he is escaping all kinds of 5 point harness then you could consider an impact shield, e.g. the Cybex Pallas range.