Id just take it slow and steady. He wont even notice if you choose to top up a bottle with a bit of cow's milk and increase it gradually - that way he gradually gets used to the tatse and the texture changes slowly for him. Pretty sure my dd didnt even notice a difference the day we offerd her a full cup of cow's milk because we diluted it so slowly over time.
If he's waking during the night for bottles maybe try increasing his solids thru out the day (include snacks between meals - yoghurt,cheese,dried fruit,bickies). Try and give him a hearty dinner (pasta is good) followed by the bottle just b4 bed. Remembering cow's milk wont give that "full feeling" that formula does , so you will probably need to up the solids when you decide to introduce cow's milk to his bottles. By then he might drop a bottle because its replaced with morning/arvo tea like a yoghurt/fruit ect.
Maybe have a sippy cup/bottle of water at his level that he can help himself to thru out the day.
goodluck sounds like ur doing a great job
Aww Thank you so much! That all makes sense, thank you for all the awesome tips