22 months old still doesn't eat solids

Our 22 months old boy still doesn't eat solids and he doesn't even show interest in trying it. He was born prematurely at 26+4 (3 months early), he is otherwise fine and active, talking (words only), recognising most of the letters and counting up to 20. When he was about 2 months old (still in the NICU), during a breastfeeding attempt he had a bad aspiration where his saturation dropped significantly (below 30%) and the medical team had to intervene and put him back to breathing support. This has certainly traumatized my wife (she was unable to breastfeed after this), and we aren't sure if it have left a mark in him as well. He is otherwise having a varied diet (my wife cooks for him), and before feeding, we always purée his food. He is being fed, but recently shown some interest in trying to feed himself, but inconsistent about it (sometimes he doesn't want to do it). We have tried everything we have been suggested (by health professionals or family alike): giving him finger food on his table, eat the same as him and show him how we chew, not puréeing his food fine, let it have bits, etc. Sadly, whenever he encounters a solid piece bigger than a size of a pea, he starts gagging and afterwards refusing to eat from that food. For breakfast, he is eating muesli and that's about the biggest solid food piece he can manage... Any ideas or suggestions? Should we wait for him to be ready, or is there anything we can do to help him?