Hi Kate,
I have 4 guinea pigs, and have kept g.ps. for 16 years so I know them pretty well. They are nervous little creatures and it is no good expecting them to come to you, you have to make all the running. If your son picks them up - separately - every day, and just spends time cuddling and stroking them, they will eventually calm down. My last one, Molly who has just died aged seven, was so tame she used to cuddle into my neck and I could do anything with her. With perseverence and as they get older, it will get better. Good luck. They are lovely little animals.