This is a very touchy situation....It's like being caught between a rock & a hard place. Either way you go, someone's bound to get upset. If you stand up to them (his family) & make a comment, they'll likely get frustrated with you, saying something to the effect of, "He's my/our son. I/we had him & know what's good for him!" or something along those lines.
However, if you don't try to enlighten them in some way, both you & he end up hurt.

You are very good to be so caring & for doing the best you can to help him as you are. I truthfully cannot think of any good ideas at the moment, other than talking to him in confidence about it or perhaps giving his mom/family a nutritional pamphlet (that you might be able to obtain from his dietitian or endo). Another good idea is perhaps having him bolus a little more/less (dependent upon how much & what types of foods he eats while there). That said, maybe someone else can offer some other ideas? I'll try to think of other good ways to handle it, but as I said, it's a very touchy situation...
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ALL my love, Carwy & Best wishes for a healthy new beginning!
Saying prayers for him & all our friends, every day.
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"We ALL have our own opinions & beliefs...But I'd appreciate folks a he** of a lot more if they didn't belittle one another when they disagreed!!!"
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Pumps & Meters Used:
MM506,7,8,11 & 12, Cozmo, Animas 1200 & 1250
Many
A1C: 6.4
Type I 26yrs, pumping 12