It does sound a little funny. I would have never expected this problem. Poor boy is gonna be fly sprayed and swat-Ed like crazy, plus in during the day. Might have to put him on steroids for his bites.
Thus is a problem I NEVER thought I'd have!
Charlie Brown (1994 bay TB X gelding)
White Star (2004 grey TB gelding)
Mine had a giant, swollen thing last summer I called the vet in a panic, a course of dex & smz's & banamine did the trick. I have never seen anything like it, kind of wonder if it was a tick bite (or two!) rather than flies or mosquitos...good luck.
Poor baby! We had a few weeks of horrendous flies last year and my one gelding refused to drop to pee, clever boy he didn't want his penis being bitten while he peed.... sure was strange to watch a stream come right out of his sheath though.
Try "Calm Coat", its not super long lasting, but repels the noseeums, mosquitos and ticks pretty durn good for the few hours it does work, and its absolutely safe enough for the most sensitive private areas. Its what I use on my boys' sheaths, sheath openings, inner thighs, under the tail and ears/face. Its NOT cheap, but I've been using the same bottle 2x per week for 2 years now. A little goes a long way.
Ask yourself: "Can I do anything about this?"
If you can, do it. If you can't... then you can't and leave it at that. Worrying achieves nothing but stress.
I second calm coat. It's expensive, doesn't last too long, but it really works and doesn't irritate sensitive tissue. It helps if you put it in a little tiny pump spray bottle, you end up using less and covering more.