I think it depends. Is he happy scooting on his tummy? If so then you probably don't need a walker. We used a walker for our oldest daughter because she HATED being on her tummy (she would scream and scream until we came to rescue her). She was happiest standing up and actually learned to cruise around on furniture very young (7 months).
She wanted to explore the house, couldn't walk yet, hated being on her tummy - so she was one unhappy baby until we got her a walker. She loved that thing and would fly around the house in it (we didn't have any stairs in our house and had wood floors so it was safe enough for her). She learned to walk really early, and only AFTER she learned to walk did she learn to crawl.
We never used a walker for our younger daughter because she loved crawling and being on her tummy, and seemed in no hurry to learn to walk. Also she would climb anything, so putting her in something on wheels wasn't such a great idea.
Anyhow, that was our experience. I hope it helps you!
NOTE: after reading a few other responses, I feel I have to chime in... as I mentioned, my older daughter never crawled. She's now 5 and just graduated from Kindergarten, and can read at a second grade level, can do some pretty complex math in her head, and is in a ballet class with third graders - the kid can do jumps that I couldn't do until jr. high. She has a huge vocabulary and I can't get her to shut up. Some kids just never crawl, but it's not necessarily anything to worry about!