What you're seeing is perfectly normal behavior for what your son has experienced. My oldest daughter was 2 when she was placed with us and I was "mommy" #4 in a 4 month period. There were several things I did even at that age to help with transitions. I started out with going back to the gym and leaving her in day care next door for increasing periods of time. I always left her with "Mommy is right next door and this is when I will be back." Then it's important that you're not late and when I picked her up I made a point of saying "See Mommy's back. This Mommy will always come back." After the first few times, we made a little game of "What do you know about this Mommy?" "Mommy always comes back".
For longer periods, I would do a simple "calendar". For day care, it was a list of the days activities and we'd look at which activity was the last before I picked her up. On rare occasions, I needed to be gone for the weekend. I did a simple table on Word with a column for each day listing her usual activities and inserted when Mommy leaves and when Mommy comes back. It sat at her place at the dining table and she crossed off each activity during her meals.
If the transition is still occurring (meaning he's still living with your parents), try transitioning by doing caregivng in each others' homes. I woulld go to the Foster mother's house in the morning, give her a bath, do her hair, get her dressed and take her to her morning activities. Sometimes we would take her for a weekend, but her foster mother would do the same activities at our house. It makes it easier to understand that the adults are all working together and he's not losing any of them. Look at discipline as well. All three caregivers (incl. daycare) should be on the same page with rules governing behavior. Are your parents more lenient with him? If so, the stress may be coming from the lack of set limits. Children need to know exactly where the limits are during transitions.
Also, try sending something of yours to your parents house and vise versa. When you send your item, try asking him to keep it safe and bring it back when he comes again. The more you can give him a sense of permanence, the easier the transition will be.
It's not an easy road and it won't be a quick fix. At age 7, we still need to be careful with her transitions to new situations. But once your son understands that the adults in his life aren't going anywhere -- just trading places -- it will get better.
HTH
S. in San Jose