If you truly think he is hungry then continue with the milk but I would think the 5am WU is more routine/development based. Possibly OT or teething discomfort or habit causing a 5am WU but he is then soothed by the milk and going back to sleep.
Really with 3 bottles of 8oz he is at the high end of milk intake (24oz) during the day and this night feed is not necessary or desirable for his age. The guidance is a max of 20oz per day inclusive of dairy solids. That 5am bottle would be taking him up to more like 32oz plus whatever butter, cheese, yogurt and milk is used in his solids meals. So you can see it is a bit on the high side really.
Here's a link with milk intake info:
http://babywhispererforums.com/index.php?topic=67569.0Are you able to sooth him back to sleep without feeding (ie shush/pat or your usual method of comforting)?
Do you feel it could be habit, teething pain or routine based WU?