Well for me, the difference isnt really that large compared to a "nilla" relationship.
If it doesnt work well it just doesnt.
Since you havent actually gotten the collar yet either then it is even less of a hassle imho.
I have been in plenty of relationships both nilla and D/s and all those endings were "good" ones so if you are just honest and upfront it shouldnt be too much of a hardship on any of you two. I am pretty sure that even he can see that if he cant really be there for you as is "needed" to actually be in a "real" relationship then it really isnt gonna work and most people is clever enough to see that. (Yes that includes guys )
Stay strong and it will be for the best.