Key words to avoid in baby food (please note the list is not comprehensive so, if in doubt avoid:
Glutten: Avoid anything that contains the words trigo and gluten. Other presentations of glutten are not part of the average Spanish diet, so as long as you keep away of pastries, bread, wheat pasta, some cereals, some sausages and cooked meats things will be fine (ham and sausages may have added milk powder and glutten, though this is quite regular in the UK, it's not that bad in Spain, however, beware of the salty content so don't use them too often)
Normally we carry a box of glutten free biscuits and order chicken or meat on the grill (pollo a la plancha, cordero a la plancha, ternera a la plancha) with vegetables on the side. Boiled potatoes (patatas hervidas) or glutten free biscuits (your own) can provide the carbs content needed.
Dairy (lacteos):
Possibly present in bread, biscuits, chips, some frozen vegetables and some hams and sausages. Also in yoghurt, cheese, butter, margarine, ice cream, chocolate, etc. anything with a prefix ?lact? (lactosa, lacto globulina, except ?acido lactico?), caseina, leche, and whey (I don't know how to translate whey .
To be honest, considering how young the baby is I will keep to the vegetables and glutten free biscuits. I don't know how long you are going for but you may take enough cereal for the breakfast, biscuits, the formula he uses (distribute the tins between the different bags and be sure to put at least one tin in your hand luggage), a bag of gluten free pasta. And complement with fruits and vegetables. That will get you through. Another good thing to bear in mind is that you will be in holidays which means that things won't always be as predictable as usual, so go with the flow, and don't stress much about the food, he can make up for the changes when he is back at home.
Have a good time