I used to feed it as a staple, but i was warned that it isn’t as healthy as regular hay. I believe it has more calcium, which can lead to chalky pee, and is higher protein. I don’t think it will necessarily cause problems, but I would keep it as a treat if he is happily eating timothy. You don’t want to get to a stage where it’s all he’ll eat, and he’s picky about it.
I just add a small sprinkle to the top of their hay a couple of times a week. I also use herb and forage mixes every day, which is a good healthy way to introduce some more variety. So things like dried blackcurrant leaves, dandelion leaves, plantain and petals.
If you’re in the UK, pets at home does very reasonable own brand forage mixes like this:
http://www.petsathome.com/shop/en/pets/merch-groups/feature/woodlands-treats/woodlands-rabbit-food-170g
There’s also loads of websites that specialise in just things dried forage 