You are the foster parent and you pup is your foster child. Give him reassurance, stroke him and murmur to him in a calming low voice - lower tembres would tell him that it is nothing wrong with those things, keep him as close to yourself as you can, stop and ask him gently to sit between your feet, walk with him in the busy places where are many other people and dogs - when there are many objects of distraction he would be confused, but not feaful; if isolated, he would be stronger concentrated on the objects in the distance. Continue your walks daily, try to avoid giving him bad experiences - don't allow people to touch him ( you can train "Meeting the strangers" later, and avoid staying with other dogs longer than 1 minute, just walk away and call your puppy after yourself. They are vulnerable at this age, he would be braver only you show him that you are capable to shoo away dangerous beasts. Train fireworks now, before this year New Year night, if your puppy is shy, one of reasons could be his ear drums - too sensitive.
But I can warn you. Fear in the early age might indicate agressive inclinations in the future. He is a puppy and has to be protected, than he would be a naughty disobedient teenager between, say, 7 months and two years of age, and would turn into a protector himself after - don't expect any real agression before he is two and half. Agressive GSDs need a lot of caressing, petting, praising. The only difference between fear and revelation of agression is amount of adrenaline in the blood, agression is simply a fear that dog has managed to overcome in himself. If you manage to keep him away from any dog fights and human agression ( including verbal agression ) before he is two, you, probably, would never know what does it mean - to have an agressive GSD.