Although I am tempted to encourage you to criticize the player in question as the rules lawyer for attempting to throw the paladin's immunity card, I would really check the rules on this particular point.
Especially if you told an ooc that you were aware of the implication, unless you mastered the rules yourself, he would rightly feel betrayed if you remove a "he's a paladin but I'm the DM and it's a plot point so I declare by pure fiat that he's cursing himself anyway. "
The potential problem of metagaming is just as important. This guy has already shown a slight tendency to legislate by throwing you immediately the paladin thing right after, and pushing the evidence to see if anything funny happened.
To be quite frank, I think the template is already ready for you in this case and that he already knows very well what has happened.
The chances are that your player is already a little metagaming just how much he pushes the question. Assuming the dice roll was aimed at something his character could not ignore, he should not increase his suspicion of suspicion beyond just throwing 1.
This is also one of the reasons why many dice rolls are done secretly by the DM. This particular roller probably should have been one of them.
I like to put the role in the role play, so, since the player is already suspicious, ask him if his character is just as eager to find out, then let him play the role of the detective. Make him discover by himself whether he is cursed or not. Play the holy paranoia of his character.
Investigative roles and the like are nice recipes for polarized intrigue and arouse the curiosity of the players as their character is a great way to start an investigation plot.
If the player embodies the suspicions of his character, especially with a minimum of meta-metamorphosis, I would even be moved to give him bonuses on his detective skills tests.
This is a DMG question, not a question from MM. The MM only speaks to you about the real effect. Whether or not you find a clue to the character's player in question is a DM problem.