First thing to check - are the straps at the right height? With this seat the straps and headrest adjust together, the headrest should just be resting on his shoulders. If it's too low or too high then the straps will be easier to escape. Check this first, as you don't want an added product to mask a poor fit.
Don't make the straps TOO tight as conversely it can make them want to escape more.
Check for a big gap at his tummy - it's normal for there to be a gap here, but if there's a huge one it might be he still needs an insert. At 2, this is unlikely though. If you need an insert then he needs to be rear facing.
Make sure when you sit him in the seat you get his bum right to the back of the seat. If he's slouched forward, that will create a gap at the tummy. Check headrest/harness height with his bum back and back straight against the backrest.
Make sure you have the gummy pads on the straps, that they are the right way around (check the product photos) and right at the top of the straps and they are sitting flat over his chest (curved over the shoulder).
If you're still having issues he might be sucking his tummy in. You can try to deal with this behaviourally - use whatever techniques you normally would such as telling off, distraction, redirection, praise for sitting nicely, reasoning if he will follow it.
If you have another seat available, sometimes swapping to a different brand or model of seat is less easy to escape, sometimes just doing this for a couple of weeks breaks the habit/association and then you can swap the seats back (e.g. if you'd swapped with a friend/sibling/borrowed one or are using a less-safe spare seat).
You can try adding something bulky to his wrists, especially if it's "gummy" like a child's bulky watch, or some silicone type bangles, those charity bracelets that used to be popular etc. However he may just take these off.
Last resort is adding something to the straps to make it less easy to escape. There are various options, the most commonly suggested are 5pointplus, which is a kind of velcro thing that wraps around behind them and blocks the sides of the harness so they can't access the gap they create by sucking their tummy in. The second is Besafe Belt Collector which is a flexible bit of softish plastic that holds the straps together just under the pads. You can also get various anti-escape clips on amazon, halfords etc like houdini stop or other unbranded ones, these are less recommended as they have hard plastic which could potentially cause a hazard in an accident. Another option is putting them in some clothing like a hoodie or button up shirt and closing the clothing over the harness so it isn't accessible.
V last resort might be changing to a different seat permanently, changing to FF (if rear facing) - sometimes this makes no difference, and then they're just less safe for no reason. Obviously changing to a different seat is expensive too. However the Joie seats are a bit notorious for being easy to escape because the pads are so bulky. You can't officially use them without the pads as the grippy bits are part of the crash protection.