I think as a parent they would be horrified that it had been troubling you all this time.
If it were me I'd probably sit him down and reassure him that whatever is said that it makes no difference as to him being your dad but, it's something which has troubled you for years and with all the health issues you have, it's something that you just don't feel you can ignore any more.
Remember, it may be a painful thing for him to raise as well, for example, if there is something in it and he isn't sure or has some suspicion, whether warranted or not.
Bear in mind, he might not know and the issue may not end there and you may not get an answer you want at first, possibly just further and more confirmed doubt.
Whatever the outcome, it would appear he has brought you up as hi own, whether he is your biological father or not.