Your post is very focused on you and what you will be missing out on. What about your son's needs? Do you think he'll be ready for school next September?
If you don't feel he'll be ready, for whatever reasons, I'd look into deferring rather than messing about with part time timetables which means he'll miss out on lots of things in school (social things, as well as the obvious academics).
With all due respect, wanting an extra day with him to have days out is not a reason to disrupt your son's schooling as it would be to satisfy your needs rather than his. What would you do once he's compulsory school age? If you want greater flexibility, you need to look into home schooling to be honest.
Being able to spend less time with your child once they start school is sadly something most parents have to face and get use to, especially those who work evenings. It's part of them growing up. The years before they start school are indeed precious and need to be made the most of but you adapt. That's what weekends and holidays are for, quality family time.
You still have a whole year of regular days out before he starts school. Make the most of it, rather than dwelling on it.
Obviously I don't know what you do for work, but is there any chance you can look into changing your shift pattern next year to work more during the school day and be around more in the evenings?