Hi Cathy . . .
I've been taking Metformin (among other things) for Type II Diabetes for almost 15 years. Until quite recently I was taking 1000 mg in the morning and again in the evening but have now cut it down to 1000 in the morning and only 500 in the evening because as I lose weight, my diabetes becomes easier to control (I've also cut back on my other diabetes meds and my insulin, too).
Metformin is probably the only oral diabetes med that does NOT cause weight gain. The only side effect I've ever had is a bit of an increase in gas. Although I know loose stools are noted as being a possible side effect, I've never had that problem. Sorry, but I have no ides how it might interact with the thryroid meds.
Hope this helps . . . have a great moving and shaking kind of day, Chickie . . . and best wishes for your son.