In Texas, the public preschools are only for those children with disabilities or significant developmental delay of some sort (e.g. speech or language delay). However, there are Head Start programs for financially disadvantaged kids, and they are low cost or free. I would contact VISD and ask them about it. They would have the right info.
As far as being necessary, it really isn't. He needs to know his ABCs, basic colors and shapes, numbers through 10, and how to count to 20. Those things (and more) are easily taught at home if you're concerned about money. Many children go to kindergarten without having gone to preschool, and they do just fine. Also, many kids come in not knowing all of those things as well, and the teachers usually have them where their peers are by the end of the year.
If you're just wanting a part-time gig for him to be able to spend time with other kids and be in a group environment, you might look into a church-based program. It would be 2 or 3 days per week and would be less expensive than a 5 day program.
Good luck! =)