First of all, Yusuf (Joseph) (peace be upon him) is/was a prophet of Allah (the almighty).
The Quran said:
“And We bestowed upon him (Ibrahim [Abraham]) Ishaque (Isaac) and Ya’qub (Jacob), each of them We guided, and before him, We guided Nuh (Noah), and among his progeny Dawud (David), Sulaiman (Solomon), Ayub (Job), Yusuf (Joseph), Musa (Moses), and Harun (Aaron). Thus do We reward the good-doers.”-(The Quran 6:84).
Secondly, Allah (the almighty) doesn’t have/had any son, nor any offspring.
It is clearly mentioned in the Quran:
“Allah has not taken any son, nor has there ever been with Him any deity. [If there had been], then each deity would have taken what it created, and some of them would have sought to overcome others. Exalted is Allah above what they describe [concerning Him].”-(The Quran 23:91).
‘And say: “All the praises and thanks be to Allah, Who has not begotten a son (nor an offspring), and Who has no partner in (His) Dominion, nor He is low to have a Wali (helper, protector or supporter). And magnify Him with all the magnificence, [Allahu-Akbar (Allah is the Greatest)].”-(The Quran 17:111).
“Say: He is Allah, the One and Only (God, Lord, and creator), ‘Allah-us-Samad’ (The Self-Sufficient Master, Whom all creatures need, He neither eats nor drinks, He is eternal, He is the one who doesn’t have death or end). He neither begets nor is born, And there is none co-equal or comparable unto Him.”-(The Quran 112).
In conclusion, Yusuf (Joseph) is/was not the son of Allah (the almighty), rather he is/was the prophet of Allah (the almighty).