There are huge red flags for me reading this post. They have been together 5 years, yet as soon as there is talk of some sort of commitment he is making excuses. There comes a point in any relationship where it's either time to take the relationship forward or break up.
His excuse is pathetic to be quite honest. How does he know how they will get on unless he tries? Life is all about experience & the fact that he isn't even willing to give it a chance says a heck of a lot to me. When the push comes to the shove he's backing out.
Your daughter is old enough to make her own choices but she needs to be realistic here, she needs to look at the way he's dealing with this situation. If he truly cared for her he would be happy that they were moving forward.
My advice would be to tell your daughter to run as fast as she can in the opposite direction. Seriously this guy reeks of commitment issues & he'll only bring her nothing but pain & misery.