First things first, have you/his owner had him checked out recently and ensured he isn't hurting anywhere? Always advisable to get physio and saddler to check them regularly when being brought back into work as they can change shape quite quickly, and canter, particularly the transition into canter is a classic point when they will go 'ow!' as they have to lift their back and push from the hocks to make the transition, TBs in particular can be prone to spine issues so with what you describe as an 'explosion' my first thought might be some low level pain making it harder for him leading to the overreaction.
If not, then I would probably have some lessons on him to get an experienced person to help you out, as he is green and hasn't been worked much correctly there may be lots of things going on to do with his balance, self-carriage, straightness, flexibility, impulsion, rhythm (or lack of same) and the 'fix' may be different for different horses and riders, its hard to say without seeing him. Lots of people will say 'work on his balance/strength/flexibility in walk and trot and the canter will come', and I don't think that's wrong per se, but in my experience with my mare who massively struggled with the canter when I first had her, you actually have to do lots of canter work to improve the canter. Mine is a cobby/native type not a TB but her tendency is to motorbike around dangerously unbalanced before falling into a horrible rushy trot/tranter with legs everywhere, and is also a very anxious/stressy type so sounds not dissimilar to your boy.
By 'do lots of canter' I don't mean laps and laps of the school, quite the opposite. I don't think I cantered more than a single 20m circle on my mare for over a year. I mean lots of short bursts of canter (sorry I know the transition sounds like the worst part, it was on my mare too so heart sunk when I realised I had to do lots of them over and over again!), always always trying to ask for a proper downwards transition before she fell out of canter by herself or rushed off in a panic, even if that meant doing only a few strides of canter at first. Then slowing/containing the mad rushy trot back to a normal/balanced situation, getting her to relax a bit again, then asking her to go back up into canter. Gradually I worked towards longer and longer periods of canter, with her being able to bend and let go of the tension, rather than fixing her neck and stressing, and being able to carry herself rather than leaning on my hands. Lots of praise for even a tiny bit of improvement. I'd repeat the process as many times as I could over maybe a 15-20 minute session, but stop as soon as I could feel her tiring or getting over anxious, as then I knew we wouldn't get any further progress.
I know it sounds a bit mad but my instructor had us doing shoulder fore in canter down the long side - I thought she'd lost her mind asking me to do a medium-level test move on a horse that couldn't canter a circle
, but gaining control of her shoulders helped me contain the rhythm and got her off her forehand and carrying herself a bit more - not saying it looked like Valegro and Charlotte or anything but it helped me! I guess what I am saying is sometimes asking for something a bit harder can help with the easier stuff in a bizarre way. But it was certainly a long process requiring lots of patience and sometimes it was a bit one step forward two steps back and I did need lots of help from my instructor.
I think also any work you can do to build up his strength and fitness out hacking will really help you, ideally lots of hill work, cantering up a hill with you in a light seat will encourage him to use his back end and lift his back and also should help give you some brakes
. Lots of horses just don't ever hack out brilliantly alone I'm afraid so if you are able to find some hacking buddies somehow that would really help. Otherwise I think again patience and persistence with the napping/anxiety when he's alone is the only way, keep sending him forwards when he does it and massively praise forward movement even if it's more/faster than you wanted, sit quietly through the paddies is the best I can suggest?