Poor little bub!
And poor Mom!
Two thoughts come to mind:
If he hasn't got any teeth yet, it's possible he's teething. And if he DOES have teeth, maybe he's getting MORE! Try some Infant Motrin at bedtime and see if it makes a difference - if so, you're on to something, although make sure it's not any other source of pain like an ear infection.
The other thought is a little more obscure: since he's has two chest infections, has he been on antibiotics? If so, ask your ped about probiotics. You should be able to get some in a powder form at a store like Whole Foods or other health food chain - they even make some especially for babies - that you can mix right in with a bottle or with food. The antibiotics can mess up the intestinal flora, which in turn affects digestion (and the immune system, believe it or not), and while he may be distracted during the day from either tummy pain or teething, at night when it's quiet, he's more likely to notice pain or discomfort.
OK, I did think of one other thing: Is he OK when you're out of his line of sight, like if you leave the room he's in during the day? 9 months is prime time for separation anxiety for some kids, but some go earlier or later. If this is it, you're kinda stuck for maybe a week till it passes.
HTH - let us know!
