hi sis @amethyst028 - i think you're doing a great job trying so well to be as objective as possible for your son's well-being... it's not easy to do what you are doing so first off, give yourself a big pat on the back for trying to see things from a different perspective, to make sure you do what is best for your son, and not be swayed by your anger over this unfair situation.
i think only you will be able to make the best decision on how to handle all this, even after hearing different views. You are the only one fully cognizant of your son's ability to handle heartbreak (from broken promises from the dad), your son's resiliency from letdowns, or your son's ability to discern/determine the hardships of life. He might appreciate the burger and toys from dad now and think you are not spoiling him when you're paying his full tuition, but in time, he will recognize the truth. It's difficult for children/teens to understand these things as they are more focused on 'growing up' and not the real world.
siguro a few questions to ask your son: is he being treated well when he is with his dad? does he feel like his dad delivers on his promises? etc? I always found (and was taught this) that sometimes it's best to ask a person questions instead in order to help them make a decision, instead of recommending a path for them.. so ask those 'telling' questions to your son?
As for suing your ex -- that will be a lot of work and it is up to you if you are willing to do it, how it will affect you, your son or how you can do it without affecting your son. I'm sure you will find the best course of action.
I also wanted to share that my father left us when i was a kid. In the early years (10 plus years) of the separation, my dad was still 'around' but was not an active father - he pretended to be active but he never delivered. My mom never spoke ill of him and never made us choose/restrict us. As adults, all of us siblings realized how much of a liar my dad was and how much respect we have for our mom because in spite of her circumstances, she raised us to believe that true love and goodness exist, and that our circumstances were a challenge, not a disability. So in the end we believed we were capable of loving and being loved unconditionally. I'm the only girl and I was not jaded by it, i always knew a good man is out there (and i'm now married to one). As for my brothers, they seem to have adapted/modeled behaviors from other male role models around us (my mom's siblings), and all turned out fine. One incident i recall though, when i was a teen and i wanted "things" like my classmates, i resented my mom for not buying them for me. She said it's my dad's fault. Instead of believing her (at the time), i just got annoyed with her and thought she's the reason why dad left (her behavior and being so selfish about money). Obviously i know now how wrong i was. But my point is, as a teen, i could not have seen the truth because i was too stubborn and my priorities were to grow up, fit in at school, etc.
I hope you find clarity in your situation sis. feel free to update here or share more details/vent. baka makatulong kame in terms of "what would we do if we were in your shoes."
Also congrats on your engagement to a wonderful man. It sounds like your son will have a good role model around him, so if the dad is not active in the picture, LET IT GO na lang : ) Trust me, seeing my bros now - worth it!