It could be anything, even teething.. at which time they sometimes nurse/feed less. Both my kids did that. He chews on the spout... and this can mean he is teething or simply not interested in drinking or not hungry.
Keep in mind, that although at this age they are on solids...for the 1st year of life, breastmilk/formula is the PRIMARY source of nutrition for a baby.
At this age, generally, is it better to offer the bottle BEFORE giving any solids. Giving solids beforehand, is usually don't to "wean" from bottle/breast. Solids first makes them full... thus, they won't be interesting in their bottle feeding.
Try not to fill up on juice... or at least cut it in half with water. They don't need the refined sugars... and it can be a "habit" you don't want to "train" them on, yet.
Water is fine... but if this is filling him up and thus interfering with his formula feeding... then don't offer this first. If he is simply not interested in water or other liquids... that is perfectly fine. For some babies, it can be an "acquired" taste to get used to. In the case of breastfed babies for example, they nurse for 1 year and water does not supplant this.
Perhaps, just keep offering him his formula STILL in a baby bottle... perhaps he is simply not ready for a sippy-cup. Every baby is different. At this age, they don't "have to" use a sippy cup, unless they are fully adept at it. Nothing wrong with using a baby bottle... it is for nutrition sake AND you want to make sure he is not losing intake. My clue is, you said he just "chews on the sippy spout thing.." So to me, he is probably not yet ready for it.
Ask your Pediatrician how many ounces he should be drinking for his age and how often. Also see if his weight gain is fine? If it is, then that's probably fine. If he is not gaining weight or losing weight... then that is a concern. But yes, at least he has "wet" diapers... although not "soaked" as it might usually be. Keep an eye on it though... per
dehydration. Dehydration can also be detected by their fontanel... if it is "depressed" it can be due to dehydration for example.
In any event, both my kids behaved this way with nursing/or bottle feeding when they were teething. And it passed and was temporary. At least it seems they are sleeping well at night. Good for you! And them. You are very lucky!
Ask you Pediatrician though.
Take care,
~Susan