Have you tried just running with him without the bike first to see how he is in a run vs. just walking?
After I knew Frankie was ok running with me (because when he runs freely he's a maniac), then I introduced the bike.
Just like you... we walked next to the bike first, then I stradled the bike and walked that way, then slowly started to peddle very very slowly all in one session. I did this for 3-4 days in a row and once he got it, we just took off. After I knew he could do it, I ordered the walky dog bike leash and we LOVE it