Urgent: cat constipation

Long story short: I have rescued a kitten and I've been trying to get his weight up. Up until now he's had very bad diarrhea, but today he is very constipated. He hasn't tried drinking for over 12 hours and now has been trying to go to the bathroom multiple times and looks as if he's in pain when he tries. Only very small bits and he meows in pain. He's going to the vet tomorrow but I'm wondering if this is very serious? I introduced a new food yesterday but only a small bit. He didn't have water overnight (dog got into garage accidentally and ate/drank everything). His brother passed away very unexpectedly today. He wasn't showing any signs of sickness and was going to be wormed this week. Is this the start of something serious?

He needs to get to the vet ASAP because he might have a blockage.  In the meantime, try offering a bit of plain canned pumpkin mixed with some plain meat baby food to see if that helps.  How old is this kitten?

It's really hard to tell. Constipation can have so many causes such as an obstruction from swallowing an object to loss of nerve function. It is good you are taking him in tomorrow. The only recommendation I can make for sure at this point is to feed wet food if you aren't already. Make sure that water is available from now on, hydration is key to treating constipation; some people fight constipation with sub-q's. Let us know what the vet is able to find. He will likely feel  the colon and maybe want x-rays.