The biggest, biggest thing to work on at this age is socialization. So unless your DH is going to go to a playgroup and have regular (1 or 2x per week) playdates, I think you should think seriously about this.
Also, check with your local school district on kindergarten readiness and when he would be eligible to go. If he's going to be eligible to K at 5, then you might want to leave him right where he is for now. If he has another year, and you as a family can commit to getting him ready (letter/sound recognition, etc.) then go for it.
Check with your preschool teachers, too, especially for your son's class & find out what they think. Tell them honestly that finances are super tight, but that you don't want to short-change your son's development. Ask them if they think reducing days will impact K for him. Also, ask if they have a financial assistance. The preschool we send our son to has a couple of scholarships available based on financial need.
My DH is a partial SAHD, and I have to say that while I have been disappointed in his doing social things (DH Is an introvert, and I think the idea of going to a "mommies" group gave him the heeby-jeebies, LOL), he has wholeheartedly embraced preschool for our son, and realizes that the school is better suited for helping socialization than he was. He has done a tremendous job, and it fills my heart with joy to know that our son is so attached to him. But I also recognize that if our roles were reversed, we'd do things differently, and as long as DS is happy & healthy and moving in the right directional developmentally, it will be okay if DH doesn't do things the way I would.