Make sure to make him burp after every meal that is once you feed him don't make him to lie down, instead put him on your shoulders and gently tap on his back for atleast 15-30 mins which will help in his digestion
Show him in early morning and evening sunlight so he'll gain his daily dose of
vitamin D which will also help in improving his immunity and growth
I would advise you to breastfeed in sitting posture which again will help a lot in digestion
Other than this if you have any doubts or more information kindly consult with me on practo because I need more information to thoroughly analyze and help you