There are two factors to consider when starting infants on soft foods. One is that the extrusion reflex with which they are born-the reflex that makes them push anything solid out of their mouths-should be gone. Otherwise they cannot move food into the back of their mouth to swallow it but just let it dribble onto their chin.
The other is that the child both needs food nutritionally and is ready to accept it. Between 4 and 6 months, infants need additional iron, which is readily available in commercial rice cereal for babies. They really do not need anything besides breast milk or formula before that. One concern is that starting infants on foods before 4 to 6 months may make them more prone to allergic reactions, especially if there is a history of allergies in the family.
Having said all that, it comes down to each individual baby and the choices that are right for him. If he is a big baby, is trying to mimic your eating behaviors, and asking for more feeding than just the breast or bottle, you certainly will not be doing harm by offering him a tablespoon or two of rice cereal thinned with breast milk or formula. Don't expect that this will make him sleep longer at night, and avoid giving him a variety of other foods until he is at least 4 months.