Strawberries are a fairly common allergen. The others common ones in babies and small children are:
milk (you know about that one already though)
soya (lots of babies with a cow's milk allergy also cannot tolerate soya) and other pulses
shellfish (but also other fish like cod)
What does he eat?
What are his favourite foods? These are the ones mostly like to be causing a problem (the short term endorphin realise is addictive and makes you crave more of it even although long term it's harming your body).
Have you tried cutting out all food and just giving him milk for a few days and see what happens? (There's no reason why you can't do that easily at his age. Most of my kids didn't start having solids until between 13 and 15 months, milk only is perfectly fine until the age of 2).
What does he eat before bed?
What is his poo like? Can you see food in it? Does it smell normal? Does he have a red ring around his bottom?
How often does he have a bath? What toiletries do you use?
Where does he sleep? How old is his mattress? How often do you wash his bedding and how?