Hi Kimberly,
I wouldn't worry about the brown spots too much, as long as it's a fresh avocado. They do turn brown very quickly, but are still OK to eat. Yes, you can mix with formula, in fact this is a good idea, as it will 'ease' him into a new flavour.
At his age, I don't think you need to worry about it being his main meal - he's still getting everything he needs from his milk. I would advise that as it's a new food to feed it to him on it's own, just so you know if he has a reaction that it's definitely the avocado (not that that's likely, it's just the advice, I think).
HTH