NC
I can sympathise, and just because you have questioned yourself about it, does not necessarily mean that you have not bonded with him or don't love him.
I have those thoughts regularly about our youngest, she looks so different to the rest of us, completely different colouring, has the sweetest nature of anyone I know (unlike the rest of us), has talents none of the rest of us have. I once broached the subject with my DH and he was furious with me at even thinking it, so I have never brought it up again, but I think about it regularly and shudder at the potential outcome if that was the case.
I would be absolutely terrified to get a DNA test in case my worst fears were true, she is my everything, I absolutely adore her and whatever her DNA may be or not, I am still her Mum and she is my life.
Like others here who have had those thoughts, she almost died at birth, I didn't see her for the first 6 hours, DH saw her for the first time 4 hours after her birth, and by the time we did see her, she was hooked up to machines and ventilator, and we couldn't even see her little face or hold her. She spent 6 weeks in NICU where there were another 2 close calls when we thought we would lose her.
I never associated her traumatic start in life, with the ongoing thoughts of her being switched at birth, but it does make sense now and actually gives me a lot of comfort and reassurance. Especially to realise I am not the only NICU mom here to feel that.
For you, if it really is something you want to do, I believe it is possible to do discreet DNA tests, i.e. a lock of hair, his toothbrush, he would not know you were doing it. It can all be done by post. You would obviously need his father to agree to do a DNA test too, it would put your mind at rest at least.