A look over the odds and latest polls
All he needs to do is to make Jadot a nice offer and he will be in strong running. On the assumption he accrues all the Jadot voteshare (and none of anyone else’s, such as Poutou’s), he will be 5% ahead of 3rd-placed Le Pen (Ipsos), 0.5% behind her and 2.5% ahead of 4th-placed Pécresse (Ifop), and 2% behind her and 3% ahead of Pécresse (OpinionWay). Zemmour and Pécresse would be out of the running, going by all three polls.
The Wikipedia table of French Presidential polls is here.
Taking this puts him in equal 2nd place with Le Pen on 17%, with Le Pen on 13% and Zemmour on 12%.
He needs to portray Macron as a right-wing candidate, which may not be difficult.
That said, Zemmour is likely to fall further and his share may be divided between Le Pen and abstention.
Jadot withdrew last time, supporting Hamon rather than Mélenchon. Hidalgo doesn’t seem to be much of a player yet despite her position as Mayor of Paris – and given that she is behind Jadot in the polls this bit of history won’t repeat itself. Personally if I were Mélenchon I would ask Jadot what he wants and then offer it to him (which is presumably what Hamon did last time). Jadot clearly has the skill to take account of ongoing developments and choose not to act as a headbanger, which is more than can be said about several of the other candidates.