Babies are very skeptic about ingesting solid foods as they have only been fed on breast milk or formula until now. So, they may push the food out of their mouths or may purse their lips together so that you cannot get the spoon or the food inside their mouth. This is just natural.
Babies younger than four months have more permeable gut that can absorb whole proteins too and thus, the risks of developing a food allergy is much greater at that age. So, do not try to give him solids then.
Breast milk or formula provides enough nutrition for the babies younger than 6 months of age but older babies need additional calories and nutrients that can be supplemented by solid foods. One year olds may need about 400 extra calories from solids while 2 year olds may need about 600 calories.
If you try to force feed a baby, he or she may not be able to use the tongue to push the food properly from front to back and may gag on it.
Infants that are fed cereal before 3 months old are at a higher risk of developing celiac disease (a serious intolerance of wheat protein) and children who are fed cereal before three months of age and after seven months of age are at greater risk for diabetes.
Try to make baby eat only one or two teaspoon of any new food or at least taste it. The experience with eating should be positive for the baby.
You have to be patient and know that this process has to be slow and force should never be used to make the baby open the mouth. This may mean that baby is still not ready to take in solid foods or it may take as many as 20 tries before he trusts a particular food to ingest it.
You may start introducing baby to solid foods between the ages of 4 to 6 months as the reflex of pushing food outside the mouth with the tongue disappears, though you may delay it a bit if you are undergoing too much pressure of work or stress, so that you can be more patient with the baby.