Corky said:I prefer the fit of a flight jacket which is a size or two larger than my suit size.
It is possible that your Dad might also prefer a larger jacket.
Best of luck...
You can stretch horse, but in my experience (using hot water and wearing it till dry) you're not likely to get more than 1" extra out of any part of the jacket - more likely 1/2". Better than nothing, but it's hardly going to give you a whole new fit.
What won't stretch permanently is anyplace with sturdy stitching . Sleeves can come down a bit, even waistlines, but collars and arm holes? Forget it. They'll expand when wet and then pull back to size as it dries.
Goatskin is pretty much a no-go - it's just too resilient. All you can do is shrink it!