Huge hugs Nina,
I could have written your post a few days ago. DD was doing exactly the same thing and ours lasted about 3 weeks, but yesterday and today has been a fast improvement.
From reading your post, I would say that it is almost certain that it is down to teething. DD cut her formula in half the other day and only had about 10oz, and I could not get any more in her. Have you tried teething gel? We rub a bit on her gums a few mins before a feed and this seemed to really help. We also used paracetamol to ease the discomfort and hep with the grouchiness.
To help keep her intake up, I ended up doing a little AP, which I know is not the best way to go, but wanted to make sure she had a decent feed in the evening. I would wait until she had fallen asleep and then give her the rest of her evening bottle to make sure that her tummy was full and she had a good nights sleep.
With regards to the spoon refusal, it is not uncommon at this age that they do want to try and self feed. I offer DD something that she can eat as finger foods, and while she is playing with it and concentrating with that, I am spooning her dinner into her mouth. It does make things a little easier especially if she does not see the spoon comming and generally she would bat it away and I would end up covered in food.
I would also just keep a close eye on him. With you saying that he seems extra tired, just keep an eye on him incase he is coming down with something. He may just be extra tired as he is not sleeping as well at night due to the teething, but just keep an eye out.
Also, what size teat are you using? Do you think you could go up a stage, to help the milk come out a little faster, that way he won't have to suck as much. I know they suggest a slower teat for babies that have just been weaned from BF, but he is older, so it should be ok.
HTH's