I am a big fan of quality preschools. Kids can learn a lot at home, yes, but what a good preschool should give them is not just ABCs and 123s but socialization and group activity skills that are vital for kindergarten.
A child has to enter kindergarten knowing how to listen to and obey an adult who is not a parent; how to sit and listen to a story etc. for a certain period of time; how to stop doing one activity and move to another activity when told to do so, without pouting or balking or melting down; how to interact (as well as possible) with kids who are not siblings; how to move from place to place in a group of kids safely and as quietly as needed -- not always silently, though!
I think the experience of being in a school-like setting, with new children, with adults who are not mom and dad, for more than an hour or so at time, is important preparation for kindergarten. Preschool also can help you figure out if there are any issues you need to address before kindergarten, too, like reluctance to change activities, or separation issues, or vision or hearing issues, and more.
Look at your finances and see if the sacrifices elsewhere in your budget are worth it. Each family has to make the choice, and not going to preschool does not mean he will be "behind" when he starts kindergarten by any means, but I do think it is a big help in preparing them for being away from mom in a school setting.
Especially if your area has required, full-day kindergarten, it will be much easier for him to enter full-day K if he is already used to half-day preschool, rather than starting full-day K cold turkey after being at home. I know other parents feel differently but that's my take. Good luck!