first thing I would do is to switch dog foods... and you did that....
I would look for signs of internal parasites... could be the cause of weight loss...
next.... a trip to the vet......
That being said... I have a male Lab just like yours.... so long legged and skinny.... and he may eat two cups of food tonight and not eat again for two days..... he dose clean up all the household waste... including bread and potatoes and anything elce that has food value.... normal people have a compost pile somewhere in the yard... I have a Labrador and he is still skinny.
