My son is now 3 yrs old but still using bottle to drink milk. The first thing when he wakes up is calling for milk. He can drink very fast. He drinks 3 bottles (8 oz) of milk every day. I haven't pushed him to change to cup yet. I remember when my eldest kid threw away her last bottle into the bin by herself was around 5 years old. I think it has to do it gradually.
Do you know why your son is drinking too slow by bottle? Is it because of the hole or just is he enjoying the drinking process?
My son can finish it by bottle within 2 minutes. I use the largest size of pin (Pigeon brand) to enlarge the hole. Apart from milk, he uses cup for drinking water or juices.