Hmmm. I assume he's had blood tests to see the level of inflammation in his body? Is he hypermobile? Have they considered steroids rather than bog standard Ibuprofen or Naproxen that'll likely make him feel like puking his guts up after a few days? Is there history of autoimmune diseases in the family, like Psoriasis, Crohns', AS, RA, PsA or suchlike? It's very common to say to kids that they're just growing (I wish that phrase was fucking illegal, it isn't normal to be in pain) when they've actually got something that would almost completely disappear into remission if given the correct medication.
In the meantime, however, I'll look at it from the point of view that I get almost identical issues (with a diagnosis of Psoriatic Arthritis) when unmedicated.
Look at the quality of his bed and mattress - children's/singles are generally of fucking atrocious quality. To have muscle and joint pain and then have to lie down on the nasty things that come with single beds feels like you have somebody pressing their knuckles into a hundred thousand bruises. And when you eventually wake up, assuming you've made it all through the night without waking up constantly due to pain, you feel rather like you've been put in a cement mixer together with a bag of cobblestones. A memory foam topper would be relatively inexpensive but might take some of the pressure off. An S shaped foam pillow helps to keep the head and neck in the correct position, so reduces neck, upper back and shoulder pain, along with headaches that come from the back of the head. Trying to sleep on your side can help if you use a standard pillow between the knees to take the pressure of them and the hips/sacroiliac joints.
Stretching and trying to maintain a good range of movement is very important. Pain can make you slouch and hunch over, which creates more stress on the joints - does he poke his head forward, roll his shoulders towards his chest and slouch his upper back, for example? Posture is so important. And teenagers tend to have the worst posture of all.
A good and non harmful exercise is to stand in the door way and gently use it to give a little stretch to shoulders, back and arms. Another is to lie on the floor, knees up and just move slightly to lengthen the back, neck, arms, legs down to try and reach a little more. A sharp pain = stop immediately. It should just feel like a little stretch.
Vitamin D. Vitamin D levels have been found to be very low in many, many people with autoimmune disease and can cause osteomalacia/rickets - one of the symptoms is pain. Lots of it. Most teenagers aren't that likely to eat a great deal of food that's high in Vitamin D. A multivitamin would help the levels and isn't contraindicated unless you have been specifically told that he must not have any vitamins (which I doubt). IIRC, sleep disturbance is also very common with inadequate vitamin levels.
And, no matter how hard it is, staggering and limping into the shower each morning is really good for getting things moving. Could you fit a seat under the shower? It seems like the most awful effort at the time when you wake up in pain, but something like 'Your bath is run/shower's ready, I'll give you a hand to get there' can make it less of a chore.
A mixture of those might help him feel slightly better.