He might not have developed chewing skills yet. Also there is possibility of hypersensitivity in mouth. These things are also related with speech development.
Next Steps
Consult a speech therapist
Health Tips
Start with alternating,his favourite food and food you desire him to try. Reinforce his attempts. Don't force. Have a play way method to introduce him new foods. Observe him keenly for any signs of discomfort with new food