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.
أَنَّ عَمَّهَا مِنَ الرَّضَاعَةِ يُسَمَّى أَفْلَحَ اسْتَأْذَنَ عَلَيْهَا فَحَجَبَتْهُ فَأَخْبَرَتْ رَسُولَ اللَّهِ ﷺ فَقَالَ لَهَا لاَ تَحْتَجِبِي مِنْهُ فَإِنَّهُ يَحْرُمُ مِنَ الرَّضَاعَةِ مَا يَحْرُمُ مِنَ النَّسَبِ