Firstly, well done for being such a caring 'Mama Adoptiva". I have one of those myself, as it happens (my real Mama having passed away 12 years ago). You are, I am sure, a source of joy to him every bit as much as he is a source of joy to you.
However, in the interests of perspective, these are my thoughts... it is possible that he has reached a stage in his 'human formation development' that means that he needs to detach himself from someone who is a 'particular friend' (even if non-sexual) so that he can learn to give of himself to others in an equal way.
I know that you are smarting at the sense that you should be considered a potential source of scandal... Without, by ANY means, impugning your motives, it is possible that other priests have felt 'smothered' by other women or 'monopolised' by them and want to 'wean' him off you (even if that actually turns out to by unwarranted). I have seen it happen with my own eyes, as it happens, with a priest of my acquaintance who was, sadly, driven to near-distraction by the overwhelming attentions of ladies of 'a certain age'. I gather that that sort of thing is quite common.
I do know that some Seminary formators try to guide putative priests towards a situation where they can lean on each other for support and guidance. We, on the outside, might perceive that as a form of rejection whereas they perceive it as honing the familial brotherhood of priests.
My instinct here is that it's a very old-fashioned way of doing things. Proper human formation teaches healthy functioning adult relationships between the priest and members of his flock whatever gender they are. Personally speaking, I'd probably react rather badly against this if I were ordered to cease contact with my own 'Mama Adoptiva'. I'd probably view it as an unjust and unreasonable request and make the decision about what to do with regard to following the order in the light of my own conscience, but that does rather presume that I would have freedom of communication.
I don't envy you your position. Sadly I'd say that there's very little you can do about it. Just try to follow, if you can, your seminarian friend's progress from a distance. Pray for his welfare. If he's a truly loving and loveable (proto) priest, he'll remember you when he's ordained and you'll be able to renew contact then, in an honourable and respectable way, of course!