Can I set up 2 accounts (for me and my son) on the same computer with the same e-mail address?

You can, but you will pay for each return.

ONLINE vs. DESKTOP

When you use Online TurboTax, you get one return per fee.  Each return needs its own account and user ID.   If you use the same account and user ID for a second return, you overwrite the first one and lose it forever.

When you use Desktop TurboTax, which you install from a CD or download to your own hard drive, you can prepare and e-file multiple returns for the cost of the software.  You can e-file up to 5 Federal returns—more if you mail the additional ones.  If you have multiple returns to prepare for family members, Desktop is more cost effective.

https://ttlc.intuit.com/questions/1901306-how-many-times-can-i-e-file-using-the-turbotax-cd-or-download

https://ttlc.intuit.com/questions/2942039-what-s-the-difference-between-turbotax-online-and-the-turbotax-cd-download

https://ttlc.intuit.com/questions/2750704-what-are-the-advantages-of-the-turbotax-cd-download-software-over-the-online-version

If one (or both of you) of you meets the criteria, you may be able to use the Freedom Editiion.  You must have a different account and user ID to use the Freedom Editon.

https://turbotax.intuit.com/taxfreedom

Yes.  Online is only good for 1 return.  But you can use the same email address for 5 accounts.   How to start another return in the Online version

https://ttlc.intuit.com/community/using-turbotax/help/how-do-i-start-another-return-in-turbotax-online/00/25596

But if you have more than 1 return to do it may be cheaper to buy the Desktop program which can do unlimited returns and has more features and advantages over the online version.

https://turbotax.intuit.com/personal-taxes/cd-download/ 

if you use a windows pc, 

you can use online even if your computer's operating system is windows 7 sp1

TT has issued a warning that for 2019 returns, for the desktop version, you'll need windows 10.

it is unknown at this time if they will modify this position.