Saline drops down the nose. Place him in your lap, head towards knees so he's slightly sloped down and do a couple drops per nose. Limit suctioning to just before feeds if he is having trouble eating as suctioning can cause swelling making it even harder to breathe. Sit in the bathroom with a hot shower running for the steam. Use a humidifier, especially at night.
we have the same problem and are now battling an ear infection. Tummy time does clear out some but isn't always practical as its messy, and some babies are not a fan of the tummy time. Some peppermint oil in the humidifier. Frankincense rubbed on the chest before bed. Boogie mist up the nose before feeding, and suction after the feeding.
And i never thought i'd get on this bandwagon but don't rely on the bulb sucker. Get the stinking Nose Frida. Its not as gross as it looks and its FAR more effective. Try it yourself. Use the bulb aspirator, clean him/her out and then use the Nose Frida, you'll be both pleasantly surprised and absolutely disgusted by your bounty.
And not to state the obvious but during awake time, find a way to keep them propped up as much as possible, whether in a bouncy seat an inclined bassinet, on your lap, or if they're stable enough for a bumbo or other seat.
Re: congested 3 month old