"Byls tehdy mladý muž." is much more natural. I cannot imagine your answer without a stop and starting a new sentence. "Byls mladý muž. Tehdy..."

"byls" is equivalent to "byl jsi" if you need the "jsi byl" word order, it is better to stay with these two words. Or sometimes append the s clitic to the preceding word instead, but it is more difficult to do it correctly. You cannot append it to the and of "byl" when the "jsi" is before it and not after.