I know others on here believe in the idea that sitting up is a huge sign of being ready to eat solids. I'm not so sure, but I only have two children. The first didn't sit either. He could crawl at 5.5 months, but he still couldn't sit. Sitting came about two days before he started walking at 8mo. He started eating food at 6mo.
My new baby is 7mo and sits very well, but he still isn't ready for food yet.
My suggestion is to try it if you think your baby is ready. Just give him a little bit of food and see what he does with it. If he eats it up and loves it, he's ready. If he spits it right back out and has trouble with it, wait a couple more weeks and try again.