For some reason the guys usually really don't look forward to the engagement session! I don't really understand why since for roughly 2 hours he will be pretty much all over you and up in your space and if you ask me thats usually a pretty good time for them. :P I can reassure you that he will definitely be more into it once he is there! And if he is already excited, then girl... you have a rare man on your hands and he deserves some mad praises for that!
The best way to prep him is to
1. Tell him how hot you are going to look and that I will be making him cuddle you straight the entire time...
and 2. As mentioned above, make a date night out of it! Give him something to look forward to when it is done. I don't want to call it a "reward" because this is your future husband we are talking about, not a dog... but essentially its the same thing haha!
To avoid possible frustration, plan for delays and leave early, keep it as light and pressure free as possible for the two of you! A good way to prep your man is to show him examples of my work and what to expect, this way he can see just how fun it will be if he lets it! The engagement session is our bonding, friendship building time... It will be great I promise!