maybe he is teething, some teething babies are like that, as for my son he is 16 mths too and rejects porridge so what I did was gave him yogurt, pureed or mashed food eg: pumpkin, potatoes smoothie, oats but blended, apple and such... he eats so I allow his teeth to pop out n will massage his gums to relax, once I feel he is well with no pain or discomfort then will continue with porridge or rice intake
Maybe your baby is starting to taste and that the porridge is bland. I will mix my baby’s porridge with a little bit of ikan bilis, those white ones. Grinding them.
Add different veges, meat, or fruits into the porridge. He must be looking for more flavours. So you can try new spices too.
Cereal and biscuits are a bit sweet in taste. So may be adding sweet potatoes or pumpkin in the porridge, he may like it.
Try mixing porridge with something else which your baby likes.