You can sign all the way up to the time your son steps on campus. It really depends upon what level your son is going to play and how patient you all are.
D1's and the top D2's are generally done in the fall, with a few grabbing players in the spring--especially the D2's. NAIA, D3, and D2's will grab some in the fall of senior year, but seem to wait until the spring to see who falls through the cracks and they will grab players all the way up to their college's deadline for accepting a student.
My son started getting heavily recruited by d2's in late May, and he is still getting calls as of Friday. His friend is currently being recruited by D3's.