Oh boy Jewels, that's a tough one. My official comfort level with using a rf'ing seat to the US weight limit ends at 1yr old. The only time I would discuss parental choice in regards to the US rf'ing weight limit would be with a young child who had reached the weight. 11.5mos is young when it comes to the view of how long we like to keep kids rf'ing when possible, but it's also not 6 mos or 9mos. OTOH, if the vehicle has no top tether, that's more of a concern with a younger ff'ing child.
I would have him weighed when he arrives and make sure that his weight is correct. Also find out for sure if the silverado has a tether anchor. GM will install for free, so if it doesn't have one yet, is it an option for them to get one installed before their grandson arrives?
The 30lb limit may have been political at one time, but it isn't any longer and manufacturers still haven't raised the weight limits. It's possible there is some other factor in the equation that we don't know about...
I have a few scenarios in my head, but basically what it boils down to, is if his weight is slightly under 30lbs, I'd put him rf'ing for the short amount of time he'll be here. If he's 30lbs or over but under 33lbs, I would ff him providing that g-parents have a tether anchor in their vehicle. In the absence of a tether anchor, I would be more comfortable using the seat up to the US rf'ing weight limit of 33lbs than I would be with him riding ff'ing in an untethered seat.
*I'm not sure what the manual for your MA says - the new ones say recommended to rf up to 1yr and 20lbs, but the only requirement to ff is 20lbs. Dorel warnings put it more clearly - children between 5 and 30lbs and less than 1yr old must ride rf'ing. So from a proper use standpoint, if he actually weighs 30lbs but is under 1yr old, the proper use would be ff'ing.
So anyways, that's my suggestion. Have g-parents get a TA installed if possible - or if they have a 3rd row then you could just use a seatbelt for a temporary tether anchor... then have their grandson weighed when he arrives and then install the seat rf'ing or ff'ing depending on his weight. I'd only cross into the gray zone if the grandparents are unable to get a tether anchor installed.