You can start him out on veggies from around 12 weeks old, just introduce them slowly. And only introduce one at a time, so one new veggie per week. Start off with just a small piece, keep an eye on their poop and if all is okay, increase the amount gradually over a week or so. Then try the next one. Romaine is a good veg to start on.
Pellets can be unlimited however if your bun doesn’t eat enough hay, I would definitely limit them. My bunnies were never big hay eaters (as it happens it was because I hadn’t found one yummy enough!) so I only gave my buns two tablespoons per day. That was enough for them, obviously if you do end up having to limit their pellets you’ll need to make sure you keep an eye on their weight so they don’t end up underweight.
Alfalfa hay can be fed up until 6 months but it isn’t essential – Timothy Hay or other grass hays are fine. If you do go for Alfalfa then at around 6 months you’ll need to transition to a grass hay which can be tricky as buns generally prefer the taste of alfalfa.
As for salt/mineral licks, I’ve no idea!