Is it possible that some boys just don't pre-ejaculate. I don't believe that my boyfriend ever does. I have checked multiple times to see if there is ever anything on the tip of his penis after he gets an erection and there is never anything there. Also, does pre-ejaculate always contain sperm?
Generally, men will produce pre-ejaculatory fluid. It is necessary to lubricate the urethra, balance the pH level, and clear the way for sperm to have a safe journey. Without it, ejaculation would be a much more harrowing experience for sperm, who already have a heck of a journey ahead! Certainly, some males may produce less than others, simply notice it less, or be more or less aroused from time to time. But pre-ejaculate is very much there.
As for your second question: the answer is a very careful "no". We have a few limited studies on our side showing that pre-ejaculate itself does not contain sperm, yet trace amounts have been found in expelled pre-ejaculatory fluid. The current theory is that pre-ejaculate can "flush out" residual sperm left over in the urethra from a previous ejaculation, be it sexual activity or nocturnal emission. However, it's yet unclear what measures might be taken to clear the urethra of leftover sperm before intercourse, though it's currently thought that urinating and wiping the penis before sexual activity may reduce this risk.
Scarleteen and several other sources do not advocate use of withdrawal by teens not only due the higher rates of non-compliance, but because of the limited evidence that it is an effective birth control method, and limited understanding per increasing the effectiveness.
What we do know for sure is that when withdrawal is practiced incorrectly, (and, again, there is too much disparity to clarify what exactly constitutes perfect use, so we err on the side of caution and consider this the norm) it is as little as 72% effective (28% failure rate), making it the least effective contraceptive method available next to no method at all.