what could be wrong with my dog?

I have an 8 year old standard dachshund (family pet), my grandparents watched the dog over the past two days because my mom and I went out of town. my grandparents, especially my grandpa has a habit of over feeding him with treats, and table food, and we tell him to stop because the vet told us he was a little over weight, she said that vegetables were ok in moderation, but my grandpa took that as all the time, when we tell him to stop, he says there is nothing wrong with vegetables "they aren't fatening" so whenever he comes home from over there he is taking massive poops for the next few days. so this morning my mom got up took him out and fed him, i got up a little bit later and noticed that he hadn't touched his breakfast, then he was laying on the couch and I ran my hand over him and he cried when i petted his ear, about 10 minutes later he jumps up on the other couch with me trying to get in my lap, he will sit next to us on the couch but never on us, so that was weird, then i petted him again and touched his leg and he cried, when we took him outside to go to the bathroom he was very slow, and he didn't have a normal poop, i offered him a treat when we came back in and he maybe ate 1/3rd of it and spit the rest out, what could be wrong with him? we are planning on taking him to the vet in a little bit if he keeps acting like this, i just want some idea of waht could be wrong