Sounds like your little guy isn't done playing yet and is trying to tell you he wants to keep it up. What you have to teach him is that YOU determine his length of playtime. Lily used to pull this too. It didn't last long.
When she would come to me, I would have the leash looped through the handle part and just slip it on her. She didn't realize I was doing it until she couldn't get away. Then I would walk her to the exit and then clip the leash to her collar and slip off the loop. I think it worked cause then she was away from the other dogs, the short walk expelled some of her excitement and was less stressful on her leashing her a few minutes later.
It's best to have a treat (special one for this training only) or favorite toy (used only for this training) to begin with so when he does come to you he will relate the reward with him coming, Not relate his coming to being leashed. I would periodically throughout her play, call her, to come, make her sit and treat her. I would also use the command "settle" while she was sitting and I was holding her collar, which helped me to implement her settling down when things get too outta hand with the other dogs later down the road. In no time she would come for her treat and never predicted which time I would leash her to leave. With the periodic sit, "settle" & treat, it made it easier to leash her when it was time. You may find this method will train him for future incidents where you need him to come and settle without his usual boxer antics.
This method worked for my girl. Only took maybe 3 or 4 times to get her calmed down and ready to go when I said it was time to go. She learned quick who's the boss of playtime. He with the treats ~ Rules! LOL
