What Causes Dizziness And Weakness In Legs After Being Diagnosed With Sinus Infection?

My boyfriend had a heart attack two years ago and has been on a variety of medications since. A month ago he had to be treated for a severe sinus infection. Lately, he s feeling light-headed and twice he s blacked out and called. He says his legs are weak.and they give out. Could this be a sign of dehydration (he never drinks enough water) or could it possibly be something more serious?