If there was a little icon with a person saying 'we're not worthy' a la the movie Wayne's World I would insert it here. well done for your committment so far.
There's a book called Adventures in Tandem Nursing that covers a wide range of topics. There's more about it here:
http://nursingtwo.kellymom.com/I think if you are looking to wean your eldest it might be tricky but of course with love and patience you
will be successful. Think one step at a time. I think your first step could be to remove his dependence on needing to nurse to go down for his naps. It may not be easy but I think it would help if he was less dependent on boob for comfort in this way. You could check out the gentle removal plan here for starters (it can be applied to older nurslings):
https://babywhispererforums.com/index.php?topic=52857.0You may need to substitute a few extra cuddles at other times as the weaning process goes on.
A lot will also depend on how verbal he is e.g. whether you can negotiate around nursing such as asking him to wait/offering a substitute or distraction. Can you offer exciting 'big boy' alternatives during nursing time such as TV/ new activities? I would also work on separating their nursing sessions so the eldest's eventually only becomes associated with a particular place - not the place where you nurse the 3.5 month year old.
The LLL board as mentioned have a weaning forum and a tandem nursing forum. You may find support in both places.
http://www.lalecheleague.org/vbulletin/