A downhill helmet
Get on a site of mountain bicycle accessories with him; point out something you like. He'll unknowingly point out something he likes. Then you'll know what to get him.
What has he already got?
A stick to whack squirrels as he passes them. Bonus: he can eat them as roadkill. Ask LRJerry for some roadkill recipes. He has his bf Frank around often for intimate dinners of road kill and passion. Frank can get angry if he's been "catching" all night though.
He is probably very particular about the accessories he uses and you are likely to guess wrong. Get him a bottle of good scotch to drink when he gets home from riding.
The best gift for anyone is something they really want but are reluctant to buy for themselves for whatever reason. Just because your father enjoys mountain biking doesn't mean that the best gift for him has anything to do with that. He is your father, think back. Did he ever talk about something he wanted but never got around to buying?
Would have though a very comfortable saddle is high on the comfort list. The though of travelling for many miles relying solely on the saddle stalk on which to position ones rear orifice seems a very hazardous and unwise practice to say the least.
Simple... A GIFT CERTIFICATE from his favorite local bike shop or online website that specializes in bikes, parts & accessories. Amazon immediately comes to mind followed closely by Nashbar.com.