for some reason, my bass player cant keep in time. how can i teach him to?

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I doubt you can.

A bass teacher will, if he doesnt want one, he'll learn in time.
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Can he count to 4?

If he can't count, tell him to take lessons.
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Make sure you are jamming with drums, if not a drum machine and if not AT LEAST a metronome. This will help him to find the beat. He just needs to practice.
How do you know it's not you who is playing too fast/slow?
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Half a rhythm section that cant keep time......
buy him a metronome for his birthday
hell get the idea
Have him pay more attention to the drums and accentuate notes he plays with the bass drum.
This should help him lock on the drums.
This helped me get a feel for my current drummer the first few times I played with him.

If it’s an actual timing problem he has and not just locking in with the drums then he is gonna need some long practise sessions with a metronome.
You have to figure out why he can't keep time for starters. Any musician really needs to learn how their part fits in with the rest of the band.
Is he new to playing the instrument? If he's been playing for a while then he probably just sucks and most likely you won't be able to help him. Theres a difference between being new to playing music (of course if you're new you won't be able to keep time well because you'll be too concerned with playing a sour note) and just having terrible time. Some people have terrible time and are just helpless, the same way some people are tone deaf and can't sing worth a damn even if they are only trying to match pitch.
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If he can't count properly he's not a bass player. Are the guitars drowning out the drums? That's the most common cause of a bass player having issues following along. If that's not it, tell him to spend extra time with the drummer, to learn to listen to the drummer, and to start playing deep in the pocket until hes competent enough to try venturing out of it.
Yeah I would just say metrenome and drum machine, He has to practice a lot on his own before he comes to group practices.

Make sure he knows that he should nearly always play a note exactly when the bass drum hits.

There is a lot to be said for individual practice time (woodshedding). Being in a band will not magically make you good. You have to apply yourself and take responsibility for your own playing.