This could be an STD. You should both go get tested. Include a Herpes Select IgG test. Hurry up and have him get the sores in his mouth cultured to see what it is.
This could be yeast, but you wouldn't get them from your mother. Yeast can just happen, unfortunately.
Why don't you both get tested and checked by a doc? If you are in the US, try your local Planned Parenthood, or call your local health dept and ask about their std services.
AJ