He can't help it, he's a baby and he's excited ;) he needs to learn that to go out he must have his lead on and sit nicely for it :-)
this is how
I dealt with it with all of mine.
Teach pup to
sit
on command. Never say it loudly, just a very quiet
sit
and he gets a treat, then start making him wait for the treat while sitting :)
Once he has learned this reliably, you are ready to move onto stage 2.
Get the lead out and stand quietly, let him go mad, run around, do what he likes. Don't make any noise and don't move about, pick a spot and stay there, let him come to you - this could take some time at first as he's a very excited baby boy ;)
If you wait patiently, your pup will eventually realise that he isn't getting anywhere and
will come to you. Tell him to
sit
and put his lead on, if you can't because he's just jumped up and started being excited again just wait patiently. He will soon learn that to actually go out he will need to sit to have his lead put on. If he can't or won't, go out for a short walk (just around the outside of the house will do ;) ) without him

and come back in and leave it for half an hour.
I never chase after mine to put a lead on, if they don't sit nicely, they get left behind ;) they only get left behind the once ;) then they decide for themselves that they'd rather calm down :-D