Help with High Gravity Bottling

I have a 12% RIS that was brewed 8 months ago and bottled about 6 weeks ago. This beer is for my Brother in Law and he didn't want to listen to me about force carbing in a keg then counter pressure filling the bottles. So we added priming sugar solution to the beer and bottled. Now he is pissed that there is absolutly no carbonation and wants to keg the beer now. What would be the best way to go about doing this? The beer is currently in 1 liter swingtop bottles.

1-liters can take longer than 12-16oz which is what I usually bottle in.

I make a RIS 2-3 times a year and honestly its in the bottle 6-12 months, sometimes 18.
A RIS is like a barleywine or a wine, you have to hide it away and wait

Totally agree a beer that big can take a long time to carbonate! In addition being swing tops there is the possibility that they are leaking, especially if they've been reused a few times. Spray a little star San around the caps and see if they bubble