I would like to know if you'd consider it a red flag if a person has admitted to you that they tend to drunk dial their ex's ? (like back when they were in college)

Nah... not unless this person still does this.

If they still do it, then huge red flag as it means they still want to be in contact with them for some reason.

Sounds like someone is laying the groundwork to be able to say "But I told you I did things like that...you have to forgive me..."

My "girlfriend" said something like that to me, and I would playfully answer back, "Ooooh, really, hehe, well, do that ONCE and you'll be free to call them all you want. Understand? (tweak a cheek)"

"Hehe...oh, and guess what? (with a kiss) I'm not kidding."

RED FLAG all right, it could happen to his next ex.

I would say that this is a definite sign that a person is not over their previous relationship.

You said he only did this when he was drinking. If he drinks often, then of course you have reason to be concerned. But if he doesn't engage in this type of behavior when he is sober, maybe the problem isn't with the exes, maybe his problem is with alcohol.

I have been in realtionships before and didn't realize until it was over that there were red flags everywhere. Sometimes we just don't want to acknowledge or accept them. Remember a red flag is just that... a red flag.