Many parents feel like you do , i did , its not unusual . I think 2 and a half is a bit of a rotton age to be honest , they will cry and scream over anything , mine would scream for hours because the dog looked at him
. He will be totally differant in a year.
You sound like youve lost confidence in yourself as a parent , which isnt surprising with his dad keep asking to have him full time . Im afraid i dont agree with your description of him being this fantastic dad , good dads support their pregnant partner , they dont neglect them while theyre suffering from pnd . Tell him No , its not up for discussion and have a discussion about this crying . You keep saying hes a brilliant dad but im really concerned about whats actually going on here , for a start he wants to take his son away from his mother , which is not in your sons best interests. And he has his parents support on this doesnt he . 
When its time for your son to come home what is he saying ? Is he saying " Oh ds , youve got to go back to mummys house , you wont see me for days ill miss you so much , daddy will be so sad , fuss fuss drama drama " . Because really your ex should be saying " Woo ds , its time to go to mummys now , and you can play with your loveley toys , maybe mummy will take you to the park ect ect , how exciting ".
Unless you are beating your son with a stick this is not a normal reaction and is possible a REaction to something your ex is saying or doing . What has your ex done to resolve this problem ? Was he previously emotionally abusive to you Op ? Because men who Ea their partners will often Ea theyre children too to upset their mothers . ( Nobody needs to go off on a tangent , but its worth ruling out )
Your really young , and its really really hard , but you can do this . You can still go travelling , in short bursts when hes a little older . Tell this fucker that No , hes not having him full time , that if he continues to come home upset youll have to decrease contact . When your Ds does come home screaming and snotting as they all do , remember he is 2 , screaming is their hobby . Ignore the screaming , be excited to see him , swing him round and chuck him about , be fun and exciting .
Youve lost confidance , start beleiving in yourself ( flexes arms in a shera type fashion )