Wow...the woes of the unhappily married, needy and in love as they collide with the insatiable voracity of those who long to keep all happy. This is an inevitable collision indeed...if you continue to share the road with the woeful. So the question, "Do I continue to see him as long as I keep it platonic?" can be answered with a bit more questioning. Is it possible to keep a relationship platonic when one of the two involved has already professed that he is "in love" with the other? Can only one person ("I") be responsible for "keep"ing a relationship platonic, or is a relationship indeed a two-way street? I urge you to think carefully about these questions while remaining true to your heart.
It sounds as though your heart is with your husband as you acknowledged that you are "very happy" with him. Is your marrital happiness worth jeopardizing? I think you have already realized that answer when writing that you can't "ruin" your marriage for this friend of yours. One who is really your friend would not want you to take such a risk.
As far as whether activities' appropriate placement should be that of the labeled "cheating" box, ask yourself this... Would every meeting you have with your friend and every conversation you share with your friend be meetings and conversations you would have with your husband present? If your answer is "no", then the meetings and conversations are probably not appropriate.
I would advise taking hold of your steering wheel, using your signal stat and merging onto a different road of travel. If you happen to make a pit stop at a bowling alley or poker casino where your friend is also in attendance, amicably enjoy his company while remaining true to yourself, your values and your husband. Best wishes.