Mossie, his poo sounds normal to me - does it look like yellow poster paint? Also, exploding is normal, they fire them out at a rate of knots!!
What he's doing sounds pretty typical for a 3 and a bit week old. (I'll leave it to tiktok to say that it's 'normal'). Mine was exactly the same. I found that I had to trust my body, give in to the constant feeding and mentally accept that it could be like that for a few more weeks. Once I did that it got easier. BF seems to be a mental battle as much as a physical one, once you adjust your mindset it really does become easier.
You are in the lucky position of having to do othing other than feed so I would just follow him. Let him stay attached for as long as he wants until you can't cope any longer with having a baby attached to you then either give him to dh or go for a walk with him, to have a little break.
Also, bear in mind 3 weeks is a classic growth spurt time so they do feed even more frequently (if that's possible!). My ds2 never 'finished' a feed either, until he was about 7/8 weeks, then he started to come off on his own accord more often.
At his age there is nothing wrong with comfort sucking, all he wants is to be close to you and be comforted by you.
The foremilk/hindmilk thing is explained really well here . I found the hot tap analogy really good, because I realised that when he was permanently attached he was most likely getting a lot of hindmilk. If you squeeze your breast at the beginning of a feed you'll see the milk is quite clear and watery but if you do it after he's been on for a while you can see the droplets are much creamier.
I would also say do NOT buy the Tracey Hogg book until you feel you have bf established. Her bf advice is appalling and was a contributer to me giving up bf with ds1.
Oh, and the last thing I was going to say, whoever perpetrated the myth that newborns sleep all the time never had a real baby . None of my 3 slept for the first couple of months, thetused to spend 18 hours a day awake, rather than the other wayround. I'd be fairly certain it has nothing to do with your ability to bf, it's just him, my ds1 and dd were exactly the same when I switched to ff at 4 weeks. Ds2 (who's fully bf)is now 18 weeks and has just, in the last fortnight, started napping for more than half an hour at a time. They do get there, I promise.
This really is the last thing, don't panic about weight gain too much, my ds2 took 6 weeks to regain his birthweight. If your ds is going up that's a good sign you're doing okay.