Never give up your DS OP. I've seen 2 men who were 'left' by their mothers and although both reacted very differently, it has scarred the pair of them. One is paralysed, unable to be an adult, can't even sustain an erection, is a total non-man and never will be able to have a relationship with anyone.
The other is ridiculous to the extreme about giving his DD the best, to the point of not allowing her to grow up, spoiling her and trying to make up for what he didn't have.
Your DS is 2.5, he won't remember your reasons, valid or otherwise, and your X will poison his mind against you, he already is, all your son will think is that HE wasn't important enough for YOU to stay with him. You pissing off around the world without him will fuck him up for life. You'll never get him back. Your Ex is trying to manipulate his son, that is clear. Healthy relationships don't produce this. Either that or he is playing Uncle Dad, and letting him have non-stop fun with no boring stuff. That's not fair and not realistic.
If you want to further your education, may I suggest a Degree in Common Sense
Seriously though, you can study and still be a mother, and that is what you have to do.
My X is shit. always was, always will be. I've never had any support, and only been abused and downtrodden for the last decade. OK so I am technically old enough to be your mother, even though my DS is 5, but there is no way on earth I want to go anywhere around the world without my DS. I can't wait til he's old enough to go on a mega road trip or camping in Yellowstone - well camping, erm perhaps not THAT extreme, but you get the drift. We are going to North Wales and then Ireland this summer and I am so excited.
We used to live abroad, so I am not itching to get back on a plane anytime soon, but one day he and I will go exploring together, all the places X stopped me from seeing.