Is there a lot of smog in your area?
What is the air quality there? Climate?
Is the air dry? That can cause stuffiness too.
As you said, only from April he's had a stuffy nose.
What changed... since April???
But yes, so many possibilities as all the other suggestions here, below.
The allergy meds are not working, so stop using it.
Those in itself, can cause fussiness in kids. ie: I have used Zyrtec, but hated it because it makes me tired and irritable.
Me, my nose just acts up due to dust or pollen.
Take him to a pediatric ENT.
To see his sinus structure etc.
These things are not always allergies per say.
Or it could be.
But see an ENT.
You could also try, a Neti Pot. It is a nasal rinse.
Drug stores have it or places like Whole Foods.
My Husband has sinus issues, and he uses this and it helps him.
Even my daughter, at 4 years old, would use it with his assistance.
Or if it is inflamed/irritated sinuses (which can cause stuffiness), things like Nasonex can help. But, it is a type of steroid. So some may not like to use it for kids. And it is prescription.
Or try "Little Noses," which is for kids and is a nasal decongestant. Found at drug stores. This helps my kids stuffy noses. Right away. It is nose drops.
Your son has a stuffy nose. You tried allergy meds. This does not always target the NOSE per say. So maybe, just try some nose drops. Like "Little Noses."