I strongly suggest taking the bike and your son into a bike shop. They will be happy to show him not only how to shift the gears, but why and when. Also they may even throw in some maintenance tips.
The easiest settings will be labeled 1 and the hardest the larger number. if you were too look at the bike where the chain is, the gears that are closest to the frame are the easiest and the ones far away are the hardest. In the back this means the largest gears are the easiest in the front they are opposite, smallest gears are the easiest.
There are a number of good books and many free instructional videos on youtube that show shifting principles. The biggest issue for 10 year olds is whether they can reach the levers comfortably.
You will need to remove the shrouds on wither side of the Exercise bike. You will see on each crank, or at least one of them, there is a cotter pin with a bolt The cotter pin needs removing and you will be able to turn that pedal arm to 180' to the other one. Spare parts appear to be obsolete, but you should get parts for the crank arm from a good cycle shop. Very often the cotter pin needs to be filed to fit. Tightening the nut locks it in place. There are two videos, the second one refers to cycles which is where my experience comes from.
https://www.manualslib.com/manual/2258999/Diamondback-910ic.html#manual
Take the caps off the top of spring tube and I believe you have to have a long rod, with a socket, of what ever size it may be to loosen, tor tighten either/or the bolt that tightens the spring or loosens the spring.. Some have them some don't.. Hope this helps....
The front bearings on this bike seems to be quite standard, but there is a choice between steel and aluminum or stronger bearing cones. Any bike workshop should stock them and replace in little time at such low price, it's not worth wasting time trying to do it oneself.
Bicycle manufacturers generally only print one "owners manual" version that covers all of their bikes. They are very generic in nature. If you are looking for a resource on how to fix your bike, your local library will have millions bicycle repair manuals, Amazon, Half.com and YouTube are great resources.
Most disc brakes are the same as other brakes. The cable comes down to the brake. When you pull on the cable, the shoes tighten on the disc. To adjust, loosen the bolt on the end of the cable, with a pair of pliers pull the cable tighter, re tighten the bolt.
front end stability is a function of the frame geometry and a measurement know as "trail". If you have front end wobble it could be several factors, the wheel could be out of true, it could be out of balance because of excess tire sealant that may have coagulated inside the tube. It could be that the bike was in a minor front end accident and the forks are slightly bent, thus the geometry has changed affecting the stability of the ride. It could be that the dropouts are out of alignment, most bike shops have special tools to measure this called "H" tools and if the fork is made of steel it can be safely readjusted (if it is a shock, aluminum or carbon fiber, they cannot be adjusted). The headset could also be loose. It could be any one or all of these factors put together, so the best thing to do is to take it to your local bike shop for a thorough inspection.
Sounds like you have the bearings in upside down. The flat side faces out. After the bearing comes the outside bearing race ( its cone shaped, small end faces in). Then the keyed washer, then the locknut (the thin one). Make sure the cones are not too tight (this also causes grinding). Plain washer and axle nut (the fat one) go on after its on the bike.
