You're not alone. Believe it or not, this type of situation is the norm for international families in Seville. Over the years I've taught English to a number of second-generation American, British, and Canadian children in Seville. Their level of spoken English was usually about the same as that of their Spanish peers although sometimes they understood a bit more. All of the parents started out speaking English to their child in complete confidence that he would turn out to be a balanced bilingual. It usually came as a shock when the child began to refuse to answer in English. The one-parent-one-language method works fairly well in Seville until the child is about four years old. Then he starts going to Spanish kindergarten and makes more friends outside the home. A half hour or an hour of contact a day with the English-speaking parent can't compete with six hours of Spanish school, two hours of Spanish playmates, and one hour of Spanish television every day (not to mention the Spanish grandmother). Recent research indicates that a child needs to be in contact with the minority language at least 25% of his waking hours to maintain a balance.
Balance is the most important element in child bilingualism, and once that balance is destroyed so that it becomes much more of a chore to communicate in English than in Spanish, the child is on the way to becoming a Spanish monolingual. Many dedicated parents continue to speak to their children in English even if they reply in Spanish, which is good. Passive bilingualism is better than no bilingualism, but it won't help the child learn to speak with a native accent.
A combination of lots of parental presence, establishing English as the family language (if your spouse is fluent), long trips back home, frequent visits from relatives, abundant English reading matter, English videos, and limits on Spanish TV sometimes keep the child's spoken English going. Doing a couple of years of school in an English-speaking country before the age of twelve is very effective. If your aim is for your child to be bicultural and biliterate and get good scores on the SAT or GCSE, he would need to do a large part of his schooling in English -- which in Seville means that homeschooling or private tutoring would need to take the place of the Spanish school. None of the schools advertised as bilingual or English-medium in Seville can bring a child up to a native level in English. None of these schools have more than a handful of children from English-speaking families, and the communication among the children is always in Spanish. These schools may help children from Spanish-speaking families to reach a slightly higher level of English than they would at the local English academy, but children who already know some English from their parents tend to actively forget more than they learn in order to fit in with their peer group.
Few familes have the resources to maintain their children's level of English in such an aggressively monolingual city as Seville. However, thinking of all the young people I've spoken to over the years who seemed ashamed or resentful because they didn't learn English from their parents, and considering that, because of the inflexible job market in Seville, the majority of these second-generation Americans or British will wind up back in the "old country" speaking with a foreign accent for the next 80 years, I think we should put everything we can into maintaining our children's bilingualism.