It won't look pretty, but if the fabric along the tear isn't too fragile, you can overlap the fabric a bit and then zig zag down the length of the tear. It might be too difficult to pin the sides of the fabric together but you can overlap bit by bit with your hands as you go.

I agree that the iron on will be uncomfortable. I suspect that sewing the edges together will just put more stress on fabric already fragile and worn. i know very well that some people (not me but some other people) develop an attachment to pjs and hate breaking in new ones - the old ones are so soft and comfy.

The rip looks like it goes up into the waistline casement. So you might need to rip the stitching on the casement to get the fabric straight to mend. Fabric mend patch would be too stiff there. But you could use a bit of fusable webbing to help overlap the edges a bit. There's lightweight fusable tape stuff meant for hems and the like. You don't need a lot of hold, just enough to keep it in place for stitching. It can be easier than trying to pin the overlap. After you have it tacked in place with the tape, then stitch over it. Then sew the casement back shut.

Also perhaps consider putting new pj's on the gift giving list this year :)

beesneeds have you done this successfully? The webbing never comes out right for me

Yes, I have.

What a lucky guy to have someone who will mend his pants! Not everyone is luckly enough to have someone take care of them like that.