Don't observe veil from him for he is Mahram (one with whom marriage cannot be contracted) on account of fosterage as one is Mahram on account of consanguinity.
Sahih Muslim Book 17, Hadith 12
← Previous Hadith My foster-uncle Abu'l Ja'd (kunya of Aflah) sought permission from me, which I refused. (Hisham said to me that Abu'l-Ja'd was in fact Abu'l-Qu'ais). When Allah's Apostle (ﷺ) came, I ('A'isha) informed him about it. He said: Why did you not permit him? Let your right hand or hand be besmeared with dust.

Sahih Muslim Book 17, Hadith 11 Next Hadith → Aflah b. Qu'ais sought permission from me (to enter the house), but I refused to grant him the permission, and he sent me (the message saying): I am your uncle (in the sense) that the wife of my brother has suckled you, (but still) I refused to grant him permission. There came the Messenger of Allah (ﷺ) and I made a mention of it to him, and he said: He can enter (your house), for he is your uncle.

Sahih Muslim Book 17, Hadith 13