Praise be to Allah.
Having a low sperm count and low sperm motility of the level mentioned is regarded as a fault which must be disclosed, and it is not permissible to conceal it, because of what it may lead to of not having children. Having children is one of the most important aims of marriage, so the wife has the right to have children just as the husband has the same right.
If a man gets married and is aware of this weakness, and cannot have children, that is more likely to put the wife off him and make her regard him as having deceived her.
Hence Ameer al-Mu’mineen ‘Umar ibn al-Khattaab (may Allah be pleased with him) said to the one who married a woman but could not have children: “Tell her that you are sterile and give her the choice.” Quoted in Zaad al-Ma‘aad, 5/183
It says in Masaa’il al-Imam Ahmad ibn Hanbal, narrated by Abu Ya‘qoob al-Kawsaj (no. 1282): I said: What about the man who marries a woman when he is sterile and cannot have children?
Ahmad said: I prefer the one who knows that about himself to disclose it; perhaps his wife wants to have children.
Ishaaq said the same as he said, because he cannot deceive her. End quote. Narrated by Ibn Qudaamah in al-Mughni (6/653).
Shaykh al-Islam Ibn Taymiyah and Ibn al-Qayyim favoured the view that sterility is regarded as a fault that constitutes grounds for annulment of the marriage, unlike the majority of scholars.
See: al-Mawsoo‘ah al-Fiqhiyyah, 30/268
Based on that, this young man has to tell his fiancée, then if she accepts, that is up to her.
If he hopes that the situation will improve and the problem will be resolved within a short time as mentioned, then he may delay proposing marriage until that time.
We ask Allah to guide and help us and you.
And Allah knows best.