For me, the tip seems to be the most sensitive spot.  A girlfriend and I discovered this spot together one night when we were having some fun.  If your boyfriend is also sensitive on his tip, then you can exploit his ticklishness by following these step by step instructions:
1.  First, make sure your boyfriend has an erection.  The harder, the better.
2.  Put some hand lotion or lube in the middle of one of your palms.  The lotion in your hand should be somewhere between the size of a quarter and a half dollar.
3.  With your other hand, firmly grab his erect penis just below the head.  Keep his penis pointed outward at a 90 degree angle from his body.
4.  Now place the flat of your other (lubed) palm on the tip of his penis.  Keep your hand flat.  Do NOT cup your palm.
5.  While applying firm downward pressure, move your palm in a medium to fast circular motion.  If his tip is ticklish, you should get a good reaction.  If his tip is really ticklish, he'll go through the roof.