Personally I would try giving him his milk just a little bit earlier in the evening to increase the chances of him being awake when you put him down. How much milk is he drinking throughout the day and how much food is he having? Babies only stop waking at night for milk when they're getting enough during the day. You could try to increase his food or milk intake at various times during the day, which might help.
With our daughter, we would give her a cuddle, put her in her cot, dummy in, mobile on and then leave the room. I'd sit right outside the bedroom door and then every time she started to cry, I'd go in, pat her back, offer her her dummy and put the mobile back on. I never waited specific amounts of time as far as I can remember, but you will know the difference between a murmuring cry (sometimes they do this just before they go to sleep) and a full out cry. What you want to avoid is any distressed crying. I would definitely start with going in as soon as you hear a cry and going up from there - 5 mins is quite a long time to start with I think.
I think the main thing to remember is they need to know that you'll be there for them if they need you, so that might mean actually sitting in the room with him to start with while he settles down, then move to sitting at the door and then out of the room.