OK.
My love, you had managed to heal, to get him out of your day to day, and then when you were just sitting comfortably on your new rung of the ladder this prick sends you a message that clearly was to show you that he's STILL a wanker.
He screwed up, but he is being the same twat you left.
He has not ripped your heart out, he has shown you who he is again, and that he has not and never will change. You had got past the shock of realising that there was no hope left, and then this. The text has reminded you that the hope of him ever coming good has well and truly died.
I got a missed text from my (abusive) ex. clearly not to me. I also got a message from him talking about my birthday in July. (MY birthday is in JUNE, we were together 10 years, his PREVIOUS (and cling-on) girlfriend's birthday was in July. The text was for me alright, but he couldn't be bothered to remember the date. ever.
I tore a ffing strip off him for both of these, it's a massive lack of respect and this is what your Ex has shown you, a massive lack of respect.
FWIW, after 10 years of never bothering to remember, he's not got it wrong this year... 
Tear his head off (metaphorically naturally) Tell him to NEVER text you again, that you are not one of his tumbles and that to treat the mother of his DC like this is disgusting.
The tough love bit from me? Stop expecting him to turn good, he won't. You are hurting yourself if you carry on expecting him to be nice/decent.