+1
It's ridiculous when parents act like the kid is all upset and hurt, when it's really them (the parent)

Your kid will only notice this is an issue if you want him to.

OP, my dad felt competitive towards his older siblings and my mom tried to defend him by nitpicking inequalities between the way the grandkids/nieces/nephews were all treated. It stirred up trouble and TANKED my relationship with that side of the family apologize and from now on do not compare your son to his relatives. He does not see it as a competition unless you tell him he is losing.

You are being so petty and ridiculous. Be thankful you have a MIL so close and willing to spend time with the grand kids. Some people would trade with you in a heart beat.

No one is a mind reader, and the world doesn't revolve around you and your kid (or me and mine) so don't expect people to assume what you want. Be proactive, pick up the phone NOW and fix this.

OP, are the other grandkids older? 3 can be a hard age and it may just be much easier for your MIL to interact with the older ones. I know you said your DS is timid/not difficult, but maybe your MIL is just more comfortable with kids with more independence (no potty issues, napping issues etc). Plus older kids can do more, go more places, stay out longer, etc etc. If the other grandkids are the same age or younger then that would definitely be different.