If it's me, as long as he loves his milk I'll let him drink from the bottle. Sometimes kids have attachments. Milk can also be one of it. Maybe even comfort feeding. Like my son who is 3years old he is still drinking from the bottle.
Hi. You may find this article useful: https://sg.theasianparent.com/smooth-transition-from-feeding-bottles-to-cups/
You can slowly try giving in a cup or straw mug. You won't know unless you try. Good luck!
I myself was still using bottle at 7yrs old when drinking milk ;p