I checked the 7 traditional leagues I'm in and he has a slugging between .560 to .640 in those leagues. Sorry he is underperforming. He should catch up.

The difference between 89 for HR's and 104, only amounts to an actual difference of 7.5%(only half the games at home), or maybe 3 HR's.

Since this is in a 20 team league, the 9 teams in you league you face 130 times in the year has more effect than your home field. It's possible that you may have up to 2/3's of the best righties in your division, or guys that are ground ball pitchers, and that is what is accounting for the lower HR's.