I would definitely wait till he can sit upright. If he would be flopping over in his high chair then the choking risk would be much higher if you tried to feed him. Sitting completely unsupported isn't necessary though, so if he can sit upright with a bit of help from his highchair or on your lap, that's fine.
So I would wait till he can sit upright then just try and see how you get on. In a couple of weeks, things may be very different and he may be interested in food then.
YY to antilop. DD can sit unaided but is teeny, but it has an inflated insert which helps! This thread is v reassuring. .. started weaning at 6 months and she just "picks" and very much prefers milk. Sounds like this is ok! She has haliborange drops so getting her vitamin D.
I haven't got a highchair yet as have a small flat with nowhere to store it so was going to get one after he grew out of Moses basket. But on 9th centile, I've really got my money's worth! I think I'll go for the Antilop though, never heard a bad word said against it!
Anyone tried the Ikea Leopard? Quite liked the look of that too!