The general question of 'to shoe or not' is one that you'll need to decide for yourself I'm afraid. I would advise you learn all you can about the principles of hoof health & function and the pros & cons of different approaches. Hopefully the link to the other thread in my signature will help get you started. My personal opinion is that generally speaking horses are far better off without shoes(not that that necessarily means bare), but in some situations, conventional shoes may be the best option.
Given his age, I would be extremely hesitant to shoe him yet(or ride much/hard). His feet(& back) are still maturing and I'd wait until he was fully developed first at least.
If he lives in a soft pasture, or otherwise has no way of developing strong feet by getting lots of stimulation on hard ground, he will likely always be a bit 'ouchy' on rough going at least. Therefore I'd work at building his hoof strength. Putting down 'pea gravel' in his pasture, around water & in hang out spots, & in his stable is helpful. I would also protect his feet with boots or such if going on rough trails, etc. While exposure helps feet develop, I don't think forcing them to gimp along if they're not comfortable is desireable or helpful to development either. Horses need to be able to use their feet correctly to function properly & get strong. Sand or pea gravel tends to be generally comfortable for horses regardless of the state of their hooves.
I think it's important to trim hooves frequently enough to *keep* them in functional shape, rather than allowing them to overgrow & then have to 'correct' them. Generally I find somewhere between 3-6 weeks is optimum. When you say 'weeks & weeks without needing a trim, is this because they're hardly growing or perhaps that you're waiting for them to overgrow to provide a significant amount for the farrier to remove? (I'm guessing it's not because he is in an environment that wears them down) If they're not growing much, this can be due to diet, nutrition &/or amount of exercise/hoof function.
Being flat footed can be a problem, can mean that the horse has little between internal structures and the ground. But I don't think it's necessarily so - can be natural & fine for that horse & that environment.
Re the crack in the back hoof, it may well be of little consequence - due to overgrown hooves before you got him which are growing out. If it's been there a long time though, I would be thinking the hooves are allowed to become overgrown &/or being trimmed in an imbalanced way &/or there is infection in the crack that is perpetuating it - eating away the horn as fast as it can grow down. I think it depends how it's done as to whether cutting a 'v' will help or further hinder it's healing. Also wouldn't be inclined to shoe feet with cracks, until they've grown out at least. I think that generally speaking, shoes are not a good move for unhealthy feet.
Where abouts are his feet looking bruised? Is it all feet or just one? Bruising can be not much of a deal or can be an indication of problems. Eg. If he has a small bruise somewhere, it could be that he trod on a particularly sharp stone or kicked something, but if he has bruised soleS, particularly in the toe region this could be due to thin soles providing too little protection for his corium & pedal bone. Could also be to do with trimming.
Brushing & overreaching could be natural conformation, lack of fitness, injury, inadequate or imbalanced hoof care.
Hoof pics(check out the link in my signature for tips on those) and more info on diet, environment, etc would be helpful if you want any more specific advice.