I also loved the HR longer hair that fopped over his forehead. It was so lovely to see movement in his follicles!
There's all kinds of clever hair products like Toppik that most hair stylists and balding men use (it's a bit of a disaster in rain though). I think some days he uses it, and others he clearly doesn't. He has worn fringe hairpieces/ wigs on several of his films- CP, Avengers, TDW, OLLA. You'd be surprised how many celebrities cheat with lace fringes. Perhaps they did in TNM? Otherwise it's a 'growth' treatment or shave it clean off. He probably has a few more years before he has to make a drastic decision.