Poor you Rozi. From the dental side of things i dont think it could be the teeth. Although the canines can cause more discomfort than the central incisiors. He may just have a virus that should run its course after about 3 days.
If it is constipation that is bothering him, i have got hold of these fantastic Rhuger drops for Jayde (they also work for constipation) they help with tummy problems. Made up from rhubarb and ginger. they are $25. To find out more you can phone them on 0800RHUGER.
It could also be a food allergy, so you could try taking out the last thing you added to his diet and work backwards from there. But in saying that it really does just sound like a virus.
PS re the feijoas and apples. They are quite an acidic fruit and if he has an upset tummy or isnt feeling right they may not be the best. prunes boiled in water works wonders
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Hope he is feeling better tomorrow and you catch up on some sleep.