This might be tough because of the fibres used.
Polyester won't really stretch all that much, but wool can. Unfortunately, it being only 38% may make it difficult. If it were the other way around (60/40 wool to polyester) what you can do is take a HOT shower with the cap on for a good 15-20 minutes and let it stretch over your head, then let it dry with the shape/form from the hot shower.
But don't take my advice as you're talking to a guy who can't even sew a button onto his own shirt.
