Technically you only need 100% of the family size with the immigrants added in to determine family size for the K-1. So in your case the count would be you and him, so 2. But the embassy has a lot of discretion with K-1's and keeping them from being a public charge. Given 125% of poverty level is required to get through the AOS, they normally use the 125% number.
Get the I-134 from your mother, you do not know if they will reject co-sponsors at the his embassy. I'm assuming you still intend to try. And bring the I-864's to. Don't plan for failure, but plan to do everything you can to get through this. You can't go 100% on what someone on the phone said from the embassy. I've seen a lot of bad information given out many times.
Just checked the USCIS website and our 129-f has been approved by CSC!!! (just under a month from when i filed) I haven't received anything by mail yet, but I assume since it was an approval I should be getting my NOA-2 hard copy by mail soon! Thanks to everyone on visa journey for your help up until now. Wishing everyone here fast approvals!
Now to figure out what happens now.... ??? Just more waiting?