You will need to use a nasal aspirator to relieve the congestion, as well as a bulb syringe. What kind of nasal drops are you using? Do not use anything but water or saline solution for babies unless directed by a doctor.
When he is eating, he will gasp because he cannot breathe through his nose and suck at the same time. Use the bulb syringe just before he feeds to minimize the gasping.
If he is not eating well or seems lethargic, you should take him to the doctor. She could also have an allergy to the formula you are feeding her...milk based products can cause congestion.