MIxed breed

Turbo originally came to me from my grandson, who is handicapped and struggling because the dog wasn't adjusting well to his wheelchair.  He asked me to keep Turbo for a little while, with no clear timeline.  It's now been six months, and Turbo is still with me.  He's about 30 - 35 pounds I am guessing.  Unfortunately, he bit my landlord's dog, and he has begun showing aggression toward other dogs during visits.  A couple days go he attacked again, and when I tried to separate them, I ended up getting seriously bitten-though not by Turbo.  I have a chihuahua and he is good with her, but I am afraid if he turns on her, it would be catastrophic.  I tried psychological and reward method, but he just doesn't get it.  I cannot afford professional training but that would be my solution.  He has shown no aggression to kids but likes to jump on them.  This breaks my heart because all he wants is love.  Turbo is not neutered.