It is now recommended that babies are not started on solids before 6 months. Times change. 30 years ago I started my kids on solids at 5-6 months - under much pressure from doctors and health visitors who recommended 3 months. Now they have finally caught up with the research that shows baby's digestive system is just not ready for solids so soon.
Try starting with very ripe banana - it is one of the easiest things for a baby to digest, and just needs mashing with a fork. Once he can manage that, cook some oats in water (1 dessert spoon oats to 1/2 cup water, microwave for 30 sec) and add to mashed banana for a great breakfast..