Hi Arial,
I know you feel worried but try to relax. Is your lo happy and alert? If so, try to relax while you try to sort this feeding out. I agree with the others on the scales, unless you weigh a lo at the same time in the day, wearing nothing, on the same scale, you can't get a 100% acurate read. I always say some days my dd feels heavy, some light, and it often depends on if she's had a diaper!
I have a few questions for you...
1) Is your ds on EBM bottles fully or do you bf any of his feeds? If you bf any feeds he may be "protesting" the bottle, as well, you can't measure the bf, so are you sure of the quantity he's taking?
2) Have you eliminated all tastes of solids, even the tiny amount of cereal? At this age, I would until the milk is increased, even the smallest amount.
3) Is your MIL taking care of him in the day while you are not there? (you mentioned your MIL, so I thought I'd ask) If she is, are you sure she's not giving any solids or formula without you knowing? I only ask because if I put myself in that situation back when my dd was younger I think my mom would be sneak-feeding solids, she used to push it as she didn't agree with me waiting 6 mths (her generation gave it at 10 weeks!), she also doesn't understand the benefits of bfing (again, a generation thing, her doc told her she couldn't because we were twins

)
4) Is he given any other liquids at all (even water)? Any additional liquid could fill the tummy, limiting the amount of milk accepted.
If he is only taking the bottles of EBM and you are sure of the quantity have you thought about switching the bottle you are using? Trying one with a larger opening? I don't know much about bottles but I've read about people trying different ones until their los like one.
Again, try to not worry too much. Count his diapers, as pp mentioned. Whenever I used to worry about my dd not taking enough because I couldn't measure I would count her wet diapers (look for 6 full urine ones) and that would help me to relax and realise she was feeding fine.