I agree that painting anything on the soles is not helpful. He needs to *grow* thicker soles. If he is bruising just in the pasture, this is not helpful, let alone nice for him & I would advise protecting/supporting his feet until they are stronger/thicker. It sounds as if he's in a bad way & I would forget about attempting to 'transition' him to go bare until you have been able to first rehabilitate his feet to a healthy state. (That's not at all meaning I think putting shoes on him would be helpful or a good idea BTW)
Boots are generally not great to be left on the horse long term, but if necessary they can be. If everything else is in order, you *should* find that with a bit of support & more exercise without risk of sole bruises, his soles should thicken up pretty soon well enough to be paddock sound bare at least. I would choose a type such as Easyboots which sit below the hairline, to avoid rubbing. He'll need gaiters with them too, to keep them on tho. You will need to remove the boots for at least an hour a day to clean & dry his feet. Vettec Sole Guard or hoof casts are a couple of alternatives to boots.
First & foremost, because I think it's about the biggest single factor is to ensure he has a healthy diet. Low-carb(low grain, sugar, starch) diet is essential. It is likely that the high starch diets of racehorses & intensively kept horses is the major factor in flat/thin soles. The horse's laminae have broken down because of it & they founder. Good, well balanced nutrition is also important, as there are many important nutrients that are deficient in the diet without supplementation.
Is he being well & frequently trimmed? Do you understand the principles involved in trimming & managing in regards to feet & soundness? If not, I would suggest hoofrehab.com & barehoofcare.com as 2 good places to start..... or continue your education, as we can all learn more.
If you would like more specific advice on his feet, hoof pix & info on diet & management will be helpful. Pix from near ground level, directly front- & side-on and a range of angles of solar pix, including sighting down squarely from heel to toe. Oh, clean(ish) feet on firm ground is also necessary.