Soya milk is now very easy to find in Spain - almost all of the Supermarkets sell it and the small health food shops. The best one - Provamel - costs 1.90 euros which is about £1.30 a box and supermarket brands are just under a euro or 70p.
It is called Soja or Soy - plain or with calcio or cereales - with calcium or cereals although the cereal one in not that nice.
There is also Bebida de Arroz - this is rice milk and Bebida de Almendra is Almond milk.
The word for sugar is Azucar though the ones I buy don't contain sugar - usually Provamel.
You won't have a problem finding it - I only drink soya milk and they now sell it in Plus which is one of the cheapest supermarkets like Netto.

thank you so much for replying that has put my mind at rest now that I wont have a problem getting milk for max. We are coming out to the Costa Del Sol on friday and are really looking forward to it now. The sun has started to shine here today which has helped get into the holiday mood, thanks again Michelle x

They also have soy milk espcially for infants /babies next in the baby food sections in most of the supermarkets.