I am not saying you are the same, but a friend of mine who I suspect is also autistic gets over obsessive over friends, any friend he makes he will call, text and even message online far too much, then if they one day don't answer the phone he gets all upset. If he organises going out it will be a huge deal to him and he will often repeatedly keep ringing the person about it beforehand, but it often isn't such a big thing to an NT friend. The problem is his obsessiveness actually causes them to back away and he has lost lots of potential friends this way. Only this morning he met a new potential friend, he's already called him 3 times today and I have tried to tell him not to call him so much when he was going to call him yet again, he is also obsessively looking online for a suitable day out with his potential new friend and he wants to keep ringing him about it, this is over the top when he's only just met a potential friend today and if he doesn't back off, the person is likely to get annoyed and apprehensive over the new friendship, then if he still persists he is likely to walk away completely.
If you are calling your friend too often, try to give him some space, don't repeatedly keep ringing if he's not answering, at least leave it a while in between tries or send a friendly text message, then leave it for the day. I know it's easier said than done. There's could be loads of reasons why he didn't go, something probably came up or perhaps he simply changed his mind, but some people feel really awkward speaking to someone they're letting down, especially if they think they'll be upset and they may just ignore them instead, I know this isn't a nice thing to do, but it's common human nature and some people are naturally more reliable than others. If he doesn't answer tomorrow send him another friendly text about something nice and don't mention him letting you down, he is more likely to talk to you if he doesn't think you're going to be still upset about it, I learnt this from experience with some people.
If he happens to be a naturally unreliable person, then you probably won't be-able to change this, you will have to take him at face value if you wish to remain friends, just learn not to rely on him and if you do arrange to go out, be prepared to be let down and have backup plans if possible or if you know someone else who is more reliable invite them too so you can still go without him.
I hope you sorts things out and rearrange an alternative day that he does turn up to, or alternatively you have a good day out with someone else.