I love MK and believe that I am in touch with my inner child, but Fantasyland isn't my favorite either. We no longer have small children; my youngest is 14. We still go to WDW twice a year on average though and MK is usually still a two-day park for us. Of course we do go at a somewhat slower pace these days at the advanced old age of almost 50.
In Adventureland...we still enjoy climbing the steps to the Swiss Family Treehouse if it isn't too hot. Makes DH and I feel proud that we're still able to do it! Then there's Pirates and we always grab a Dole Whip Float and take it onto the Jungle Cruise with us.
Frontierland is one of my favorite sections. Our must-do's there are Splash Mountain (we're old enough to remember Song of the South). BTMR and again, if the weather is pleasant, we usually go out to Tom Sawyer Island.
In Liberty Square we always do the Haunted Mansion and then grab the best (if not the only!) sweet tea in all of WDW at Columbia Harbour House. He might enjoy Liberty Square Riverboat. We used to do it almost every trip, but then my DD pointed out that unpleasant smell we always commented on was the smell of feet! We usually skip it now since I tend to agree with her. I usually also skip the Hall of Presidents, but it's for personal reasons. I'll just say that I find it to be offensive to Southerners and leave it at that. If he's never done it, I would recommend it.
In Fantasyland, PhilharMagic is also the only attraction I really like. We usually do IASM world too though, just because it feels like we should.
We generally skip Toontown altogether.
Tomorrowland is another favorite of mine. There our must-do's are Space Mountain, the TTA (several times), CoP (at least twice), Buzz's Space Ranger Spin, which we've just recently learned to love, and the Indy Speedway. I didn't personally care for Monsters Laugh Floor, but again, I would suggest he try it once.
MK isn't as much about thrills for me as it is about nostalgia. My suggestion would be to follow his lead there and allow enough time not to be rushed. I refuse to believe that anyone can't find their inner child there if they want to.
Hope you have a great time helping him look for his inner child!