Ih all seriousness, you MUST get your baby to a Certified Avian Vet or even an Exotics Vet who knows how to clip wings and get his other wing clipped to match ASAP!!! I know you want to wait for the clipped wing to just grow-out, but at your bird's age this uneven clipping can cause all kinds of permanent issues such as damage to his spine, atrophy of the muscles on one side of his spine, and most importantly at his age is that his muscle-memory is being learned and developed right now, and only clipping one wing at his age often results in the bird permanently being off-balance for the rest of his life, because he'll constantly be compensating autonomically for the other side...I've seen it many times...
And you answered your own question as to why he's so clumsy, off-balance, and why he's seemingly so nervous/anxious...The reason that he seems so frightened/startled all the time is because he's having trouble balancing himself due to the uneven wing clip, and he has already learned that when he falls he has no way of controlling his fall and he just drops to the ground like a rock, or he may even spin or fall to one side...That's the reason he's so "startled" by everything, because it's impossible for him to feel balanced so he's been constantly struggling 24/7 to be able to simply stand/perch/walk without falling over, and when he does fall he knows he can't right himself.
This is a serious problem that needs to be corrected immediately on his other wing. It's absolutely not worth waiting for his other wing to "grow-out" because at only 15-weeks it may take up to a year for that one wing to fully grow-out. So you absolutely want to go and get the other wing clipped to match the same length as the already clipped one, and that way it will be done in plenty of time for them both to molt and grow-back in during his first big molt. If you wait for the clipped wing to grow, he will be constantly struggling and OVERCOMPENSATING for it, and this could very well train his Trap muscles in his back (around his spine) and the muscles in his wings to basically "learn the wrong way" how to move...
Once you get the other wing clipped to match, he'll have full-balance again, but it may still take a week or two for him to get over being startled and anxious all the time, because he will have to learn that he will now be able to not only glide to the floor without falling, but he'll be able to do it with full control. The longer you wait to get the other wing clipped to match, the more difficult the situation will become, and the more issues he's going to have with flying, walking, perching, etc. in the future. So please, don't wait for that clipped wing to grow back, because he needs to have the other wing clipped to match immediately. It won't set you back any further as far as his wings growing back in, you will be waiting just as long for that single wing to grow back in as you will for both of them to grow back in as long as you get the other wing clipped to match the already clipped one.
***This is a HUGE deal, I know you didn't realize that is was, but you did know enough that something was very wrong with the way he's been acting startled and out of balance..Single wing-clipping is something that used to be done by breeders back in the 70's-80's and before, and because it was found to cause the exact issues your bird is already struggling with, along with often causing permanent issues with birds not being able to balance, perch, or fly as adults due to muscle atrophy and "muscle-learning" issues when they are young, it was stopped as a regular practice long ago...But there are still some very stubborn, uneducated breeders who either learn from someone who has always clipped one wing, or they just refuse to change, even though there is ample medical research evidence that shows the permanent damage clipping only one wing does to a bird...So it's not at all your fault, but you do need to get that other wing clipped ASAP...At only 15 weeks his body and brain are not only growing and developing rapidly, but they are also learning how to function, and right now his brain is fighting his muscles, with his brain wanting to work properly and his muscles having no choice but to compensate for the uneven wing clip, and this will cause permanent problems if it's not corrected immediately...