Your natural test production is the highest it will ever be right now. I remember doing 2 a days, go to work,eating fast food, partying all night and getting 4 hours of sleep. And i was known as the "jacked" kid in the gym. Point is i had poor diet, over trained, lack of sleep and i still got great results. You will get great gains naturally for your age. I'd check your form on lifts, try adding more dead lifts and squats, and check your diet.
And just because some guy in the gym says something doesn't mean it's true. And ya he wants you to buy from him so he can make a profit. That's the whole point of dealing. Because honestly you probably will come back for more gear or tell your friends about it as well. He's investing for future profits as well
See one reason why steroids get a bad rep is from teenagers using them and ****ing up their ****. I really hope you don't use them for the sake of your body not being fully developed. Think about doing it after your 21.