DD sat unaided at 5 months, but I understood that was early. DS is now 24 weeks, and still cannot sit unaided- he can sit on my lap for hours, but is v top-heavy (okay- he has cheeks the size of melons) and just keels over when I take my fingers away...
How do I know when he's genuinely ready for solids? He is waking 2 or 3 times a night atm, but I think that is linked to teething discomfort not just hunger. He is interested in our food, but lets face it- would shove anything in front of him in his mouth- it's about discovery not food.
They don't have to be able to sit completely unaided, its ok if they still need a little support. If he can sit upright with a little support, grab some food and get it into his mouth, then he's ready to start trying. If he's not ready, it will just fall out of his mouth, as it takes them a while to learn how to move it around their mouths and swallow.
DS didn't sit unaided until he was 9 months and about 2 weeks! But he could sit up in his highchair booster at 5 months without slumping over sideways and was grabbing my food (and the plate!) when on my lap so I put him in the booster when I was eating and let him join in. Food IS about discovery at first.