Is he neutered yet? Sounds a lot like what we went thru with Cutty. He was around a year when we brought home our pug, Mugsy and they loved each other right away. Something changed in him tho with us and he started getting nasty at times to the point where his adrenalin would kick in so much he would be shaking while growling. Scared me to death honestly and it was so sad. There wasnt always an obvious trigger for him and we thought it must be medical since there was no explanation otherwise. Had every test run, blood work, xrays etc.. All came back fine. We decided to neuter him hoping that may help and I have to say we saw a 99% change basically overnight. He is forever changed in small ways now, cant pick him up without his growling and other little things set him off but now he isnt scary. His growls sound scary but now there is no intent behind them whereas before he really was scaring me. Mostly when he thinks he can act up I just back him up to a spot outside of my space and make him sit and take a time out for a bit till he is over his little tantrum and thankfully it has worked for us. He just turned 5 now and is our sweet, loving, big boy still aside from his little grumbles. I now wish we had fixed him to begin with because I know he would have been a much easier going boy had we done it but we didnt know. You will get lots of great advice from everyone here on training and being the alpha etc which may help also. For us putting Cutty on his back didnt work it made him worse but most on here do it with no problems and it works for them. Hope this helps a little and if not, like I said you will get more helpful training advice from others.