I am upgrading my bike, I am a size 54 road bike and am about 5'9" tall. My stepson is wanting to try out road biking and wants to start riding my old bike with me, he is 17 years old and just hit 6 ft tall. I wanted to have him still ride my old bike, but I know it is a little small for him, what is the best way to get through the next month or two of him figuring out if he wants to stick with it, before I spend the money to buy a bike for him?

Best options to get a 15 degree seat post to push that back a little bit and then a longer stem, and be good to go for at least a trial period?

What do you think?

Thanks in advance for the help.

Don't push the saddle back to lengthen the saddle-to-handlebar distance. Set the saddle relative to the pedals, then set the handlebar relative to the saddle. If you scoot the saddle back past its ideal position relative to the pedals, pedaling the bike will be less efficient and possibly awkward or uncomfortable.

If a too-small frame can be made to work, it'll be via a longer stem and possibly a handlebar with longer reach.

+1,

Since taller frames usually (not 100% always) have the same angle seat tube as shorter ones, the saddle to pedal position would be the same for a short frame/long post vs a taller frame/shorter post. So just buy the longer post if necessary (do not skimp, trying to run a post more than very slightly (10mm MAX) beyond the minimum insertion mark)

Then you'll use a new stem, with a length and rise angle which gets the bars close to target.

You may run into a front wheel/toe overlap issue with a smaller (possibly shorter length) frame and your son's probably bigger feet. But at least he can decide whether this is for him before you commit serious dough.