The boy is too scared to say no, and doesn't want to do it.
If a person can't comfortably say or indicate "yes," at any age, then the answer is no by default.

What do you say to such a person? You say "you've been raped." The same as if he were 8 or 18 or 28 or 88 and in a position where he was scared to say "no." The only difference between a 14 year old and a 28 year old in this case is there are a lot more situations where a 14 year old may be intimidated compared to a 28 year old. It's the intimidation, not the age itself, that makes it rape.

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Of 14 year olds who have sex with adults, many are at a minimum curious and do say yes, at least the first time.


Things to consider:

What were the legal and social responsibilities, if any, of the priest?
Had the priest taken a vow of celibacy (you didn't specify if this was a Roman Catholic priest)?
What was the emotional maturity level of the 14 year old?

If we assume the priest is in a religion that doesn't ban priestly sex, doesn't ban gay sex (or assume the priest is a woman), that the local age of consent is 14 (as it is in several Western countries, although some require parental consent), and that his church membership or hierarchy doesn't care, and that if the youth (not a boy anymore at 14) is either living on his own or his parents or guardians are okay with this, I don't see the problem. You tell the boy the same thing you would if he were 18 - it's your decision.

Now, on the other hand, if it's a Roman Catholic Priest in Boston and the parents are good Catholics who would certainly not approve, you have a totally different dynamic. In this case, the priest has disregarded his obligations to the church, to the boy, and to the boy's parents, and broke civil law in the process. "What do you tell the boy" - exactly that - that the priest is someone who does not uphold his obligations. If the youth has any brains, and most do, he'll understand that the man he thinks he loves has serious moral shortcomings. As the youth grows up, he will have time to reflect on the relationship and decide for himself to what degree he was a willing partner and to what degree he was intentionally or unintentionally manipulated (in most cases with 14 year olds and adults, it's not all-one or all-the-other). Third parties should not lobby him to "accept that it was his fault" or "accept that he was a pawn," he should be able to decide that himself without undue outside influence.

A side note:
If you had a gay relationship at 14 and live in a non-gay-affirming community, both "accepting that you wanted gay sex" and "accepting that you were weak enough to be manipulated" can be emotionally difficult.