Your body will only build muscle to deal with the loads you're applying to it. More food and protein will only be absorbed in sufficient amounts to rebuild torn tissues and replenish glycogen stores. The excess will be converted to fat. No matter how much protein you eat, 200gm, 500gm per day, absolutely ZERO percent of it will show up on your body as additional muscle-mass if all you do is sit around watching TV. It's the STRESS on your body that builds up muscle.

So, the OP can change his diet, but he still won't add muscle. He needs to add high-stress workouts to his weekly routine to build up muscle. Biking won't do it because it's not enough high-resistance of a workout (the bike+rider moves when you apply muscle-force).

His diet doesn't sound that bad, 1500 calories is a little low, but he can only ride 3-days a week anyway, so it's not like he's some ultra-endurance triathlete or RAAM contestant. Adding an extra 100-200 calories of protein should be fine, and an additional 200-500 calorie carbs immediately after a ride will ensure that he doesn't take apart any existing muscle to replenish glycogen stores. About 2000-2200 calories a day should allow you to have the energy to do the strenuous workouts that'll actually build muscle.