Sounds like something my dad would have done a few years back when DD when through a period of crying a lot whenever we took her out . Some people have no patience but it's a bit gutting when it's your own family I know.
It's insecurity. Some people just really can't cope with being thought badly of in public.
The first and only time I have been out to a restaurant with a baby with my dsis, the baby was grizzling because it was bored and hungry (6months and grizzling not crying). Dsis insisted we leave and then apologised to table next door for "ruining their meal". She's still a fuck wit but has moved on to telling me my house is dirty and I am fat. (Both of which are true but not really any of her business, or likely to endear her to me).
Ignore FIL he sounds limited. Your ds is fine and all children cry in awkward places sometimes.
Next time a family meal out is suggested with FIL I hope you will remind him about his behaviour towards your DS, and decline. If he is going to get up and walk out its not supportive to your family and what message is it going to send to your DS. Until he can 'man up' and try to help I would exclude him, but then I'm mean