I get the whole thing about gonorrhoea... but the thing is... our sexuzl relationship was just me and him for more than a year? So how can it be gonorrhoea... somebody told him that it is because of too much sex... is it true?

No, that is not true. You cannot get a disease from too much sex.

One of the two of you has cheated on the other. Period. No if's, and's or but's about it.