Hi you need to to speak to him away from the shop say a near by coffee shop, so the environment is more peaceful and friendly, you can say you want to catch up with him and see how he is doing.  Chat to him about his Mum and how he is coping first as the conversation continues express that your worried about him. Yourself and other volunteers (if applicable), have noticed a change in him and that he has started swearing when frustrated close by to customers.  You understand this is a very difficult time for him and where his frustration is coming from, however you cant have volunteers swearing.  Come up with a coping strategy - maybe when he feels like this he removes himself from the shop floor and carry’s out tasks behind the scenes instead until his  frustration reduces and he feels he can return to the shop floor.  You would need to brief he shop manager and volunteers on duty with him, he could say to the shop manager I need to work in the storeroom or what ever you call the back area of the shop., they then know how he is feeling and can check in with him. I hope this helps