A couple of comments from other threads are that after 6 thrusts it gets painful/harder... Here are some ways to make it easier to use a strap on:
Change his position. If you are in a position where you are finding it hard to thrust, change it around.
If possible, switch to a position where you are standing up, such as bending someone over a desk.
A great position for newbies (and I still love it) is a man doggy style on the edge of the bed. If possible have him come a little ways off the bed (so his feet are off the bed and bend his knees more...
Have HIM push on and off the strap on in a doggy style position. That will not only allow him more control, it will give you a break.
If there isn't more than 40 lbs in weight differential, have him straddle you.
Add lube. Easier pushing...
Take away any element where you have to deal with any other body parts... if you have ankles on your shoulders, it's harder to thrust... So change the position so you don't have to do this.
Don't walk and chew gum at the same time... Unless you have a sling, don't worry about a "reach-around."
Hope this helps...
Annabel Lee