Male cat marking while standing in liter box?

My male cat about 2 years old is neutered and has been checked by a vet and cleared for any UTI. We got him a couple months ago and lately he hasn't been squatting while peeing inside his liter box causing it to go on the floor and wall. We have been thinking it is some type of marking/spraying. We do have a lot of stray cats in the neighborhood and it's impossible to prevent him from looking out the windows. We have a stress relieving collar and have been trying the Feliway (sp?) spray and he still insists on doing it. If we don't get a solution to the problem soon it'll be too much for us to handle. Please help?