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. (Using translation from Muslim 1445i)
عَمَّهَا مِنَ الرَّضَاعَةِ يُسَمَّى أَفْلَحُ اسْتَأْذَنَ عَلَيْهَا فَحَجَبَتْهُ فَأَخْبَرَتْ رَسُولَ اللهِ ﷺ فَقَالَ لَهَا لَا تَحْتَجِبِي فَإِنَّهُ يَحْرُمُ مِنَ الرَّضَاعَةِ مَا يَحْرُمُ مِنَ النَّسَبِ