Tylenol and Motrin are for pain/fever, not a decongestant or cough suppressant. They're not going to help with his runny nose or cough.
You might talk with your Dr., see if they can give you a dosing chart based on weight for children's sudafed. Ours did, but that was 3 years ago. Whenever possible, we try to dose by weight, not by age.
The reason they say children under 4 years shouldn't have Sudafed anymore is because there were too many parents over-dosing their children! The harmful side effects were in cases where the children were given too much, or given doses too often. Appropriate medication when you're sick allows you to rest and your body to heal faster. But it's for you to decide what's appropriate for your son.
The only thing that works for our son is real pseudoephedrine. When he was little, we gave him a half-dose at bed time*. He's 3-1/2 (and 34#) and we give him a full 1 t dose.
Here are the other things we'd do to make him comfy:
-Cool mist humidifier (our son loves his froggy, bought at Target for $40)
-Baby vic's on soles of feet with socks on, and on chest/neck if he'll put up with it.
-Warm bath before bed
-Folded blanket under the matress to elevate his head.
-Saline nasal spray. He won't like it, but it breaks up the mucous so it can drain! There are also Little Noses brand decongestant drops - at Target. You can't combine with another decongestant though.
-Slightly warmed veggie or chicken broth in a sippy cup
-Lots of popsicles - when he's sick is the only time he gets as many popsicles (and tv shows) as he can handle!!
(*We limited dosing to bed time when he was little. If his sinuses drained while he slept, it developed into a cough and then bronchitis - then we had to have a prescrip and use a nebulizer to break up all the gunk in his lungs! Better to just appropriately and responsibly dose preventatively than have to go thru all that later!)
Hope he's feeling better soon - so you can both have a good night's sleep!
T.