My DS was on singulair but unfortunately it wasn't effective in his asthma management so we had to me onto flixotide.
The reason we were first prescribed the tablet over the inhaler is because the inhaler has more steroids in it and can stunt the Childs growth and cause fertility problems later on.
Thankfully now, DS hasn't had any symptoms of asthma for almost 2 years and no longer needs his prevented or ventolin now. Hopefully your DS 'grows out of' his asthma too!
Good luck with getting him to take the tablet, we never had that problem.