Yes it's too late to add extra 02...Unless you like cardboard beer.
AND there's NOTHING WRONG. You fermentation is simply winding down. That's normal.
the airlock has slowed down because the largest amount of fermentation is over and there's not a lot of excess co2 that NEEDS to be vented out.....That is ALL your airlock is a valve, a vent to release EXCESS co2, it's not a magic fermentation gauge. Whether it blips or not, whether it stops or starts or not really bears little resemblance to how fermentation is going, just how much co2 is being released or not. That's it.
You don't know that your fermentation is slow, or fast for that matter, all you know is that your airlock isn't bubbling in a way you THINK it should.
The fact that bubbling has slowed down doesn't mean your fermentation has stalled, it just means that there isn't enough excess co2 for the airlock or the blowoff to need to vent. All either of those are are vents, valves to release excess co2, but it is not a direct gauge of what's happening in your fermenter. Sometimes airlocks never bubble, but they ferment just fine.
It's perfectly normal for fermentation to slow down as the most sugar is consumed initially, but that doesn't mean fermentation is done nor that fermentation is stalled.
There is still more than likely a ton of work that the yeast are still doing behind the scenes, it's just not dynamic.
Just leave your beer alone. Everything fines....and don't rely on airlock activity or even krausen formation to give you accurate info on what you beer is doing.
The only way to truly know what is going on in your fermenter is with your hydrometer.
Like I said here in my blog, which I encourage you to read, Think evaluation before action
you sure as HELL wouldn't want a doctor to start cutting on you unless he used the proper diagnostic instuments like x-rays first, right? You wouldn't want him to just take a look in your eyes briefly and say "I'm cutting into your chest first thing in the morning." You would want them to use the right diagnostic tools
before the slice and dice, right? You'd cry malpractice, I would hope, if they didn't say they were sending you for an MRI and other things before going in....
Thinking about "doing anything" like repitching, or bottling, or racking, without first taking a hydrometer reading is tantamount to the doctor deciding to cut you open without running any diagnostic tests....Taking one look at you and saying, "Yeah I'm going in." You would really want the doctor to use all means to properly diagnose what's going on?
But in your case there is nothing for you to bother diagnosing, since nothing is wrong.