Is it a sin if I don't observe hijaab in front of my adopted son? I don't want to as it makes him feel like a stranger, and hurts him.Is there any way I can avoid hijab in front of him.

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

This boy is considered as a non-Mahram to you. Therefore, if he has reached the age of puberty or nearly so, then you must observe Hijaab in front of him, and you are not permitted to be in seclusion with him, and other things that are only permissible to do in the presence of Mahrams.

It should be noted that sponsoring an orphan and taking care of him are one of the best deeds. It was narrated on the authority of Sahl ibn Sa‘d that the Messenger of Allah, sallallaahu ‘alayhi wa sallam, said, "I and the sponsor of an orphan will be like this in Paradise, and he gestured with his index and middle fingers –holding them slightly apart." [Al-Bukhari]

However, adoption is forbidden and it is not permissible for this child to be traced back to your husband, and you are not a mother to him. Rather, he must be traced back to his real father if he is known; if it is not known, then he should be traced in a general manner, such as calling him Ibn ‘Abdullah (son of Abdullah) or ibn Abdur-Rahmaan (son of Abdur-Rahmaan).

Allah Says (what means): {And he has not made your adopted sons your [true] sons. That is [merely] your saying by your mouths, but Allah says the truth, and He guides to the [right] way. Call them by [the names of] their fathers; it is more just in the sight of Allah. But if you do not know their fathers - then they are [still] your brothers in religion and those entrusted to you.} [Quran 33:4-5]

For more benefit on the virtue of sponsoring an orphan, please refer to Fataawa 3152, 17241 and 94301.

Also, for more benefit on adoption, please refer to Fataawa 139996, 241351 and 119119.

Allah knows best.