If a young woman wants to take her boyfriend riding, then find him a western saddle or an Australian working saddle with a flat seat and deep knee rolls. Adjust his stirrup leathers so that he can stand in the stirrups and leave at least an inch between him and the saddle.
Use an English bridle set with buckled reins and preferably a gentle snaffle bit.
You'll need a quiet docile broad backed cold blooded horse over which you will have some control by voice from the back of your own horse. Take along a friend along but there should be no more than three of you.
Avoid trotting, - or tell the boyfriend to stand in the stirrups - preferably teach the horse to lope.
Tell him to hold the reins loose and show him how to neck rein. (Otherwise he'll jerk the horse's mouth)
Tell him to wrap his legs around the horse.
At canter tell him to lift up and lean forwards so his weight is over the horse's centre of gravity.
Tell him to follow you at all times - don't allow his horse to come alongside.
Fit him with a riding hat and, if you have one, a broad elasticated belt support.
Find him a set of gloves.
Look for a a set of English tight stretch breeches - jeans have the seams in the wrong place.
Sort out the route for the ride ie no roads, no traffic, no barking dogs.
Don't worry about his riding style and don't nag.
Don't lose your cool, or shout or show concern in your voice.
Keep telling him he is doing fine even if he looks like a sack of potatoes.
Remember he won't be able to tolerate more than an hour in the saddle.
He hasn't got the necessary muscles, his butt will be soft, as will the underside of his thighs. He'll grip with his knees until they are sore and he will be as tense as a drum. His lower back will ache and for a few minutes after he has got off after the ride he won't be able to stand up straight.
There's a fair chance that the horse will decide to throw him off - or, if it has draft horse blood, of refusing to go forwards but don't give him a whip.
Don't assume that a horse which is docile for you will be docile for him
He'll either love it or hate it and you'll know within half an hour.
If he hates it, he'll be looking out for a girl friend who rides motor bikes.