Sour Grapes

Hi all, I'm hoping someone could help me with my grapevine. It is two years old now, and growing a nice size. I have it fastened to a trellis and planted into the soil. It gets plenty of sun and water. But the grapes are very small, like pea size and are very sour. How can I promote the fruit to grow and sweeten? Whats the best feed to use?
Thanks.

 

Good evening Rhinecourt and welcome to the site. What variety is the grape and where is it growing? Have you thinned the fruit out? :gnthb: