I also gifted him the DM Manual for his birthday - no self-respecting DM is without one.
It's a great gift but YMMV. It was invaluable for me at the beginning, can't recall using it more than once per adventure (~ 4-5 sessions) once I got a handle on it. Even that was mainly to get a quick estimation of appropriate CR if I didn't have a good encounter at hand and put good game flow over a tailored enemy selection.
Now, the Trap Compendium is the gift that keeps on giving. ;)