@Dirk - The issue was to address a temporary situation, where normally he was able to just run the pool during freezing periods coupled with his concerns about lack of ability to do the work. But he seems to be getting over that fear.
We still haven't established:
1. If that angle pipe from the filter is actually the supply line to the slide pipe. If not, he's cutting, gluing, and installing a valve/tee that may not fix the issue.
2. Whether that angle pipe has a hidden branch that feeds something else, even if it does feed the slide pipe.
3. He's apparently not ever winterized in the past. Does he want to start doing so now?
Full on winterization with a shop vac or leaf blower
rarely can be done, but most of the time a dedicated blower or larger compressor is needed. He'll need more things - return plugs, skimmer gizmos, and pool noodles to get it done. Perhaps a submersible pump to lower the water level some. Some kind of cover to keep junk accumulation minimized (rather than depending on whatever usual pool skimmer/cleaning he is used to doing now).
To amplify on
@Newdude IF the OP can find a sufficient air supply, he could fasten it to the slide line, undo the union the angle pipe goes to, just before it enters the ground, and then see if air can be forced out of that now open pipe. That would then at least tell us the answer to #1.
Worst case, without finding out before hand on #2, is that we later find what other part of the pool doesn't work when the new valve is turned off - maybe a return?? - and then have to advise on how to plug that to prevent freezing....
Sounds like in past years, with the slide working, he was content. Maybe we should be helping with fixing the connection to the slide?