All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.
First of all, we thank you for treading the path of righteousness, and we ask Allaah to increase you in guidance, piety, and goodness.
We advise you to supplicate Allaah as much as possible to make you firm on the right path. Also, you should supplicate Him to bless you with a pious husband, but there is no special supplication in this regard. Nonetheless, you may supplicate with general supplications such as the one stated in the hadeeth that was reported by An-Nasaa’i and narrated by Anas ibn Maalik . Anas ibn Maalik reported that the Prophet said to his daughter Faatimah “What prevents you from listening to my advice to you? Say on the onset of every morning and evening, 'Yaa Hayyu ya qayyoom, bi-rahmatika astagheeth, aslih li sha’ni kullahu wa la takilni ila nafsi tarfata ‘ayn.' (Which means: O Ever-Living, O Sustainer! In Your Mercy I seek relief; rectify for me all of my affairs and do not leave me to myself, even for the blink of an eye.)”
You may also supplicate with the supplication stated in the hadeeth that was narrated by Ibn Hibbaan in his Saheeh from Anas in which the Prophet said, “Allaahumma la sahla illa ma ja’altahu sahlan, wa anta taj'alul-hazna itha shi’ta sahlan. (O Allaah! There is nothing easy except what You make easy, and You make the difficult easy if it be Your will.)”
What is important is that this man who proposed to marry you is religious and has a good moral conduct, because if so, then if he loves his wife, he will honor her; and if he hates her, he will not abuse her.
For more benefit on the qualities of a righteous husband, please refer to fatwa 83767.
Indeed, you did well by performing the Istikhaarah prayer in regard to marrying him or not. If a person performs the Istikhaarah prayer, then he should pursue the matter. The result of the Istikhaarah prayer is that this matter will either be facilitated for him or made difficult for him, as he had put the matter in the Hand of Allaah.
As for the fact that his name includes an Islamic prohibition and that his mother agreed without knowing you, then you should not pay any attention to this.
Moreover, marrying him depends on you only and not on your father or your father's uncle (i.e. it is you who has the right to agree or disagree). So if you are pleased with him, consider him to be suitable as a husband, and you expect that the marital life with him will last, then it is good to inform him that you agree to marry him; otherwise, you should leave him and ask Allaah to bless you with someone else to marry you.
Finally, it should be pointed out that the name Abdus-Subhaan is not permissible because Subhaan is not one of the Names of Allaah. Shaykh Bakr Abu Zayd said in the dictionary of the forbidden expressions, “Abdus-Subhaan is not permissible because this would mean being a slave to other than (one of the Names of) Allaah, the Almighty.”
Allaah knows best.