Lorie, your son has doubled his birth weight and appears to be growing well. He is also showing the appropriate feeding cues for solid feeding, so continue what you are doing. To answer your question, as long as you have not seen any reactions to the foods you have introduced so far, you should be able to give them together without any problems. We usually encourage Moms to keep to the single foods when introducing, as you have been doing, so that you can easily identify food that may cause a reaction. This suggestion applies to mixed-grain cereals as well. Keep up the good work!
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.