Without knowing your partner, its hard to tell.
For example, my partner is very calm during serious conversations, holds eye contact well and is usually very affectionate if im upset. If he suddenly was a bit twitchy, defensive or quick to anger - it would raise alarm bells.
You know your partner best, and i assume you will be aware of what he is normally like when youve previously had these conversations.
It may be worth stating that you arent having this conversation with him in an accusational way, but the fact this training course is in OW's area has brought up past feelings and emotions and you want to make him aware of that.
Your gut will guide you as to whether you think his response if off, and leaving you to feel better or worse. If you feel worse, tell him and explain why you dont feel like his answer helped to ease your insecuritys.
Dont apologise hun, i myself have been through a similar situation with my current partner so i know how you feel. The 'leave the bastard you deserve better' comments dont help when you know you want to make things work. Dont feel like you need to 'put on a brave face' - you are feeling this way due to his actions, not yours. You are very much entitled to feel how you feel, please do not suffer in silence. He owes it to you to support, comfrot and reassure you as the person that placed the doubts there in the first place.
Another thing i will add - as easy as it is to say, try not to worry. Whatever will happen, will happen whether you spend 10 hours obsessing over it, or never let it cross your mind. If it will happen, no amount of worrying or fixating will change that - all it will do is ruin your own happiness. Deal with it if its happens, otherwise you are making yourself miserable over something that may never happen x