Green snot is usually a sign of infection. So seeing a gp might be needed.
Have you tried steam? When mine are like that I like to try a steamy shower to try get the phlegm to loosen up. I find it helps them cough it up & gets the nose running, so the mucous comes out. Its not much good when its just sitting there.
Antibiotics might be needed to help him fight it though, & if you have a good gp, he should either prescribe abs, or tell you to come back in a few days if it hasn't gotten any better. Mine likes to try to avoid abs for children.