B.C.
Nothing beats using a neti pot and rinsing with saline.
Keep him well hydrated and use a humidifier in his room at night.
Decongestants don't really help - they can make the mucus thicker and harder to deal with.
If anything, an expectorant will help keep it thinned out so it drains better (Robitussin - anything with guaifenesin in it. Don't know if Mucinex is ok for a 3 year old).
If dairy thickens his mucus, avoid dairy for awhile till he gets a bit better.