In all likelihood, his hair will probably grow back in just fine on its own as his spring coat comes in. That being said, I think the act of rubbing ANYTHING on the rubs will help (the rubbing being what helps, not the goo you put on it). If he's still wearing blankets (Assuming that's the case since it's still effing cold here!) you can try something that will help reduce friction, like Chamois Butter (for cyclists and runners). I used this a lot on my former horse who was very prone to rubs from everything. Helped keep the rubs at bay and heal the ones he had.
Bossy bib for the shoulder rub - www.bossysbibs.com and IMO you really have to be careful putting anything "salve-y" on the rubs then covering it with a sheet or blanket again. The salve will just collect debris and make it worse in my opinion as the blanket wont be able to slide and move and the stuff that you put on will just adhere to the blanket and cause a build up
I've been using Chamois Butter on mine, since he still has his blanket on (we just got another two inches last night- GRR) and I wanted to reduce friction. When the warm weather finally arrives, I'm sure he'll grow his hair back no problem... but I'll probably slather on some MTG for good measure.
I've never had huge success with MTG for rubs, but this year I experimented with putting DermaGel spray on the rubbed spots, and surprisingly enough the hair started growing back, even while still wearing blankets.
During the entire time the horse is clipped and blanketed I keep an eye on the rub areas and work on prevention. So I keep the shoulder area of the blanket clean, I keep the shoulders of the horse clean. That means spot cleaning the blanket and horse. During daily grooming sessions I spray with a fine mist of hair moisturizer (is the cheap healthy hair spray) to keep the skin from drying out. This spray has lanolin in it.
Best thing is to take the blanket off and let the new hair grow in. Another way of saying that the horses oncoming spring/summer coat will prob grow in fine and your problems over until next winter.
Buying a better fitting blanket will do wonders in rub prevention.
I agree with NRB: time for a new blanket that fits properly to prevent the rubs in the first place.
That said, I've used Eqyss Mega-Tek with great success to regrow hair when my mare skinned her withers trying to squeeze herself between the top 2 rails of her paddock fence so she could play with her buddy (not one of her brightest moments! ).
You can't make the hair grow, but you can help the baby fuzzy hairs trying to grow in stay on. I like to use cowboy magic tail detangler - rub in on the rub spots (generously) to smoothen the area so the blanket doesn't catch. In a pinch I have also used moroccan (argan) hair oil stuff... or that expensive biosilk stuff (I got a huge tub of it somewhere and will not be able to use it up on my hair within the next decade).
If this doesn't help then the bossy bibs are your best bet. Key is to reduce friction to prevent the blanket from rubbing.