i had to syringe feed my gucci for 4 months before she passed away, at first when i had nothing in i gave her creamy oat porridge by heinz, that went down very easily, you can also boil up some veggies such as carrot, parsnip, brocolli, cabbage, celery, tomatoes, peppers what ever else he likes and blend it down, i also came across something called ella's kitchen its puree baby food in a sachet with nothing added to it not even water, so if you cant make that buiy a sachet of the veg.
if you make porridge without the sugar and milk maybe add a little stweed apples made by you as the shop bought ones contain way too much sugar, try and get him to take it off a spoon if you can its safer then it wont get on his lungs but syringe feeding is fine just take it slowly and give him time to chew it, offer him water evey few syringe fulls, have you checked his teeth? gucci had malocllusion of the molars, which resulted in her front teeth overgrowing. if you need anymore help dont hesitate to get in touch
if it becomes a perm thing which hopefully it wont you may want to invest in some critical care, and pellets if you dont already use them.
when i made gucci's mix i did this 1tsp crictical care - 1tsp of mushed pellets - 1 tsp of oaty breakfast baby food - 3 to 4 tsp of puree'd veggies, mix it all up with some warm water, as piggies generally prefer it warm to cold when they are eating mush