You can introduce solids any time between 4 months to 6 months if your baby is ready. Cues to look for if your baby is ready for solids: *Head control. Your baby needs to be able to keep his head in a steady upright position. *To keep solid food in his mouth and then swallow it. *Sitting well whe supported. *Your baby's mouth and tongue develop in sync with his digestive system. You can read more about starting solids from: http://www.babycenter.com/0_introducing-solids_113.bc You can consider buying the bumbo chair to help your baby sit up right.
Strong neck control is more crucial than being able to sit. You can try feeding him solids but he will need some assistance to be in the upright position while seated. My friend who had the same issue fed her baby in the car seat and also tried having her boy in her lap. A high chair could also work with added padding to help "secure" the baby in the upright position. Hope one of these suggestions work for your baby!
You can still feed baby solids. Most of my friends let baby rest in the bouncer and feed them. When they are able to sit supported then they will change to sit to booster seats. I started feeding my baby solids at 4 months and he sat in this booster seat i got from baby fair and slotted a pillow behind him. Time to time he will slide down or fall to the side. I will readjust and continue feeding.
thanks