I am extremely worried about my puppy! Help!?

Ok so im not sure of his age and i don't really feel like doing any of the math. But a friend gave him to me because he didn't have time for him. I got him on Easter and he was 9 weeks at the time. Anyways he has been living with me since then and he just loves it since i also have younger siblings so there is usually someone outside playing with him. (i also have a three year old dog who he plays with) Well anyways he is a St. Bearnard and he hasn't been eating since last night. We gave him dewormers about a month ago because he hadn't been dewormed and might have had worms so i don't think its worms since i haven't seen any in his poop. He is usually a very happy dog always getting excited about food no matter if he sees us with broccoli it doesn't matter. He will try and eat anything that is edible(Unlike my other dog who is a very picky eater) But today since he wasn't eating i brought him some leftover meat to try to coax him into eating since he loves it and is always trying to steal some from us when we have BBQ's. He came all excited with my other dog and usually when i give them treats i tell them to sit and they do, well he sat and looked all excited as usual but when i dropped it in front of him he sniffed it and walked away. I then got another piece of meat and put it into his mouth and he started chewing very slowly. I then tried to put more food into his mouth but his teeth were barred shut after that. And he also is usually very good about coming when i call him but today and last night he would just be laying down and only look at me when i call him instead of coming to me like usual. Also, this may sound kind of gross but whenever i pet him his penis comes out and just hangs there. And it is all the time that he does that no matter who pets him. He loves it when i scratch his chest but he didn't get excited today. I can usually tell when he is excited because his penis will come out (i know its kind of gross). Im not really a fan of seeing his penis all the time but it just shows me that something is wrong, can you help me?