Again I feel a sympathy for Jack, that I don't remember from my first read. On the surface, he's obviously descending towards the homicidal maniac we know he will become, but at this point, leaving aside what we know, what has actually happened?
This is Jack's last opportunity to redirect his life. He wants to write, craves the solitude the Overlook should give him. He has the chance to prove he can do something without stuffing up.
Now his son is having vivid dreams, hallucinations even. Clearly Danny is in distress, but adults do not generally alter their plans for a child's imagination. As the reader we know Jack should get the hell out of there, but I understand his the conflict going on inside of him.
He wants the best for Danny, but trying to jet ski down the mountainside is fraught with danger, and his prospects inside the town with no possessions or money are far from certain. And then what? Once winter is over is just left with broken promises and failed dreams.