If you use a travel bag for pram or car seat you can put lots of extra bits in there and they have to take them
You could always buy formula out there. Spain is in the EU so there will be regulations in place to ensure the formula is safe. Personally I would pay for a suitcase for LO, so you can make sure you don’t have to worry about anything. Check with your airline about carry-ons, Jet2 for example don’t allow hand luggage but a 10kg checked bag is included for them.
We took full formula can with us sealed and also premade bottles. Took a flask with hot water too. We took a car seat and checked it in front of the plane (we just popped the nappies and formula in the car seat (doona) before we checked). The doona has been a lifesaver for holidays ( been away for about 5 times with 6 month old)
You can take as much pre-made formula with you as you like all airlines/security have to allow that. We recently went away and had a checked bag full of it but also had 6 of the pre-made bottles going through security. They just have to do a swab of them but you’re allowed as much as you can. You could also bring the powder form as well, not a full box but as long as it’s in a sealed bag it’s all good. We’ve done that when we have kettle and fridge in our room. For sterilising bottles when away, if you are in a hotel just ask if you can use their microwave or bring some of the cold water sterilising tablets. With regards to carry on they don’t get their own suitcase but I’ve always brought my baby’s back pack size Changing back because you are entitled to that. Are you planning on bringing a pram or car seat? One other way to get stuff for him without buying him a bag is to check your car seat at the desk but use a car seat travel bag.