Thanks for your query.You have involuntary ejaculation during sleep.This is not uncommon at this age and you need not worry about it at all.This will settle down of its own over a period of time.As regards your doubt about the size of your penis going down i would make clear that it is not possible scientifically.All other problems related to erection are due to anxiety
.So do not worry at all.Following measure will help you to boost up your confidence and getting good erection.
1) Practice regular exercise fior 45 minutes followed by meditation
for 1/2 an hour in the morning.
2) Take high protein diet rich in vegetables and fruits and vit A,C,D,E.
3)Take anti oxidants like Almonds 5-6 everyday..
4) Avoid alcohol and smoking