My ex-wife and I never used -- or on her part, even considered using, as far as I know -- the douche nozzle. The ones I've seen are bud-shaped, for lack of a better way to describe them. Maybe fluted would be a better choice, like a twist drill, only with straight flutes. The same went for a nurse-girlfriend I played with after divorcing the "ex".
Then, some years ago, I connected with a retired man whose "thing" is giving other men enemas (yes, I found him on Craigslist). His nozzle of choice is the douche variety of which we speak. I never found it comfortable going in -- in fact, it could be downright irritating until the sphincter had been breached. He's a bit ham-handed -- I know he's not conscientious about aligning any nozzle with the angle of the anus before applying pressure. But I suspect the shape of the nozzle has something to do with the discomfort, as well: it . "asks" the anus to conform to a multitude of shapes as it goes in -- not simply circular dilation, but riding the peaks and conforming to the troughs of four flutes. A simple test of the appropriateness of the fluted nozzle for enema-giving might be a quick survey of the nozzles that are original equipment, plus the aftermarket offerings: the profiles varies widely, as do diameters -- but I've not seen a fluted nozzle labeled for enema use. As for the location of the holes, which are intended to wash the walls of the vagina, I can't say I've noticed any difference in effectiveness between them and the conventional enema nozzle, which shoots a single stream forward. The combined flow capacity of the holes in a douche nozzle may be greater than the single hole of a conventional enema nozzle, so lower, not higher, bag height might be called for to get the rate of injection in the optimum range for bowel acclimation. If you're thinking that the douche nozzle holes might be covered by the walls of the rectum, I wouldn't worry -- the flutes are apparently there to provide space between the holes and surrounding tissue. Also, the solution needn't be shot into the rectum at high velocity to be effective -- we're not jetting clogged drains here. That becomes apparent when you consider that clinically recommended bag height, as I've seen it, is only 16" to 24" above the anus; similarly, the instructions that came with bulb syringes, when they were a common sight on the druggists' shelves, said "squeeze with only moderate pressure".
You're fortunate to have a boyfriend who, I get the impression, is not enema-savvy, but is trusting enough to allow you to try something new on him. If he's not acquainted with the "procedure" and its many pleasures, why heighten any initial reservations he may have by inserting something that might be less than comfortable? I'd recommend staying with the ordinary enema nozzle for starters, well-lubricated and slid in gently. Follow that with slow administering and lots of verbal assurance that he's in good hands. Done right, there'll be plenty of new sensations going on in him without making him flinch by tweaking his sphincter!