What a child finds entertaining varies a lot for individual children. My 7 year-old daughter likes toys with buttons to push, and pretty much always has. My 4 year-old son likes cars, and pretty much always has. My 18 month-old daughter likes dolls even though her siblings almost never play with them. We haven't encouraged or discouraged any particular toys with them. They just gravitate towards ones they like the most. Your best bet to figure it out for your son is just to expose him to lots of different toys and see which ones he likes. Play dates, cousins, church, or day care are good places to observe him with lots of unfamiliar toys. Also, something as simple as taking him toy shopping with you to see what catches his eye.
One place I like for buying toys "cold" is amazon.com (search for toys and games). They break it out by age group and have lots of user submitted reviews so you can see which toys worked best for other parents. A lot of the reviews have the rating broken out by different characteristics such as durability, fun, educational, etc. You can also leave comments on reviews to ask the reviewer follow up questions about a specific toy.