Why don't you take him to Babies R Us and walk through the baby toy aisle? Don't just park him in front of the display; take a few things off the shelf and see if they peak his interest.
If the baby isn't rolling yet, you can always play roly-poly with him. Put him on his tummy and place his favorite toy (or a new exciting) to his side. Shake it and make it seem really cool and attractive so that he has to roll from stomach to back to get the toy.
Although some people turn their noses up at these as baby toys, rattles are actually GREAT for this age. Around 3 months, babies really get into noticing hands, playing with fingers and might start clasping and unclasping. Give him a rattle or keys (toy, not real) for him to work on that fine motor skill of grasping and transferring objects from one hand to another.
Other good, age-appropriate recommendations include:
-bright board books with bold colors
-textured toys that make sounds
-any toys with mirrors (babysafe, not glass)
A book you might be interested in reading is "The Wonder Years" by Tanya Remer Altmann. It is published under the auspices of the AAP and serves as a great guideline for development and has fantastic suggestions for play/toys that will promote development (it WON'T speed it up - kids develop on their own time table).