First of all you have to make sure that your child is crying not because of any other systemic issues like gastric problems. Normally milk teeth start erupting after the age of 6 months. As the child is 9 months old, most probably he is getting his first tooth. During this time the infants tend to bite on everything as the gums feel itchy. So, if there is no other systemic issues then there is a chance that the child is irritated because of the itchy ( or painful) gums. Also at the teething stage infants are more prone to fever and stomach infections as they tend to insert everything in their mouth.
Next Steps
Visit a pediatrician to rule out systemic issues. Maintain hygienic environment around the child. Massage gums with gentle pressure. Antiinnflamatory medications if required ( after consulting with the doctor). Most important 'wait and watch'.