:-) Our family had a string of wedding that either my hubby or I was in... The latest included 2 of our kids as flower girl and ring bearer... The youngest was about 3... Here are some tricks I used...
LOLIPOPS!!!
Not sure how the procession will go (will you be up front already before he walks up?) If not,...Your hubby will already be up front. Both of you should have a lolipop up front already. When it is time for him to walk down the aisle have you or hubby step into the middle of the front of the aisle so child can SEE you... If he wants the comfort of parents there is only one way to go and that is front... Kneel down and hold the lolipop in your hand so he can see it but its not TOO obvious. :-) Yes, bribe him. He may run to you, but at least he's moving. lol
Also take him there and let him run around... Up and down the aisle... Practice.
Have someone stay at the back with him and you or hubby be upfront. let him practice walking up the aisle... You could even do it a couple times as people are sitting and waiting so that he is mentally prepared that people will be in the chairs.
Do not expect him to stand for long... (for us) the one who was not in the wedding would stand at the front off to the side and after the kids walked up the one bribing would then show them the other parent and that one would lead the kids around the chairs to the back to the seats or back out to the hall... So the kids could eat the suckers... (bring a bib and wipes)
Are there going to be any cousins that the kids are comfortable with? Or other moms with young kids that could perhaps occupy him during the ceremony?
Perhaps rent a hotel room nearby and ask your parents if they want a pool day... They could come to the wedding, supervise the munchkin for his duties, then wisk him off to the pool and nap time... while you continue with your duties...
Good luck and have fun!