2007-08 - Dwight Howard v. Celtics (64.7% FG, 21.7 PPG, 12 RPG) - Season (59.9%, 20.7 PPG, 14.2 RPG) - you win this one
2008-09 - Dwight Howard v. Celtics (49.1% FG, 16.8 PPG, 15.5 RPG) - Season (57.2%, 20.6 PPG, 13.8 RPG)
2009-10 - Dwight Howard v. Celtics (51.4% FG, 12.3 PPG, 14.5 RPG) - Season (61.2%, 18.3 PPG, 13.2 RPG)
Career - Dwight Howard v. Celtics (51.3% FG, 16.4 PPG, 12.5 RPG) - Overall (57.7%, 18 PPG, 12.8 RPG)
Also, in the 09-10 playoffs, Howard shot 56.7% from the field (61.2% for the season). That shooting pct goes with 10.8 RPG (13.2 in the season). In the 08-09 playoffs (Magic won the series), Howard shot 54.9% which was close to, but below his season FG% (57.2%).
And if you want to look a bit further, in career splits, he shoots .3 FGs per game more in total than he does against the Celtics, so its a negligible difference. But when you note that his scoring is down and his percentage is lower, that stands out a bit because these are big games and Dwight is doing less with same amount of shots.