tum bin dost how u know that ur semen is very thin first think and 2nd who told u that thin semen is the sign of problem and also tell us how u know that ur penis size is so small.
If semen is thick but sperm count is less then person is not able to make his wife pregnant but if ur semen is very very thin and ur sperm count is perfect then inshallah u will make ur wife pregnant very soon.
If u think ur penis size is small. then do one thing. take a scale and check ur erect penis length, if ur penis length is more than 2 inch then its fine, there is no need to take tension and other thing is that if ALLAH want to give u a child then u must get it.
So dont take any tension. Just wait till ur marriage... No Need to take any tension ok... chill dear.