Eco, if you and your DP had previously experienced a bout of infertility as you mention, it's highly unlikely he would have stopped at one attempt to deviously impregnate you, were that his intention.
It's far more likely that the period of infertility you experienced if anything created a false sense of 'security' for your DP and that caught 'in the moment' pregnancy was the last thing on his mind.
If his behaviour specific to that occasion of intercourse was 'different' from usual it could have been due to many things. The unusual setting, maybe? The holiday? The thought of you having a massage (however non sexual it was, he could have visualised anything!)? Who knows!
Try to settle your mind that your DC was in no way forced upon you, s/he wasn't. You had choice, you may have said one thing but you did another and you did take part in unprotected intercourse with your DP. I can't see how he betrayed you? You were there! If anything, you betrayed yourself. Take responsibility and stop casting blame.
I understand the wish to create the best possible family scenario for your DC, particularly following a turbulant upbringing. I understand your need to find undeniable justification for ending your relationship because of this but honestly, you don't!
It is ok to end it if it is not right, it is ok to make your own choices independant of anyone elses behaviour, inclusive of your DP's!
If you are not happy, re-set boundaries, work towards happiness with or without him but don't seek justification in an event for which no-one is to blame and which, ultimately, brought you your DC.
The headshit you are giving yourself and possibly him, about this, will have long and deep ramifications if you don't get your head around it and move on from a point of here and now, not then and there.