can I reserve of a mobile beta plan now but use/activate/pay in September 2019?

Unfortunately you cannot. You will have to pay every month until your kid goes to school on September. Is it worth the investment ? I doubt it. If you know someone that can use the plan for the next 6 months and give it back, that would be best.
However, if the pricing after beta doesn't suit you, there will be more and more deals as time goes so be patient !
Best of luck.

You can not. But activate or not, it really depends. Do you have Fizz mobile? How many plans you have for your family? For my situation, it is worth to activate it now. The main reason is that Fizz allows you to gift your data. My kid will have mobile soon (like you, Sept). For sure 2G is not enough for them. I will be able to give my data to them. If I missed the introduction price, I do not know where I can find the same package. The worst, you can cancel your plan later if you think it is not worth, or find another better deal. Only one month you waste, is it right? But you have to think about the long term, not few months.

No you cannot do this

Press “Repeat” (or cut & paste) then win 3 more points for answering the same answer. ✓ (rolling eyes)

If you want advice finding a great plan at a decent price, you can pm me. I can help you out. I don't know if I can talk about other companies here, don't want to go against the rules.
Good luck and let me know if you need advice.

@Gato, please refrain from using swear words on this forum. People have the right to answer the question as much as you do. If someone answered no before you, does that mean you can't say no and give a second confirmation to the previous poster ? We are all clients here and not all of us give correct answers, which is why we share our different opinions and validate others views here.
And what if they get 3 points for saying no ? Is it against Fizz's rules ? Did they spam yes or no to every thread ? Why does others' 3 points matter to you ? Perhaps you should take your own advice and not post hatred and yet earn your 3 points, but rather keep it for yourself.

@Daoping Wei : Did we answer your question ? If so, please choose a best answer to close this discussion. As you can see, Someone has been picking on others while not contributing at all on this discussion on both of his replies.
Thank you and wish you best of luck. If you still need help finding a suitable plan for your son, please contact me in private.
Good luck.