The survival of Leopold John would have obviously butterflied away the war of the Austrian Succession in TTL but the Habsburgs would still face significant challenges in this scenario. The existence of a male heir would mean Emperor Charles VI does not base his foreign policy in TTL on the Pragmatic Sanction of 1713. I suspect than even with a son in TTL, Charles would have to dissolve the Ostend Company to placate Great Britain. Sometime in the 1720’s, Charles would have Leopold John crowned King of Hungary and Bohemia. Charles would have also move to secure Leopold John’s election as King of the Romans. By the 1730’s, Leopold John would be the object of many marriage proposals. It is possible that he would marry either Maria Amalia of Saxony or Maria Antonia of Bavaria. A marriage alliance with either Saxony or Bavaria would be beneficial to Leopold John in his eventual pursuit of the imperial throne. The War of the Polish Succession would still basically end the same way with Augustus III gaining the Polish throne and Austria losing Naples and gaining Parma and Tuscany. Since Francis is not marrying the Austrian heiress in TTL, France has no right to claim Lorraine for Stanislaw.
After the conclusion of the War of the Polish Succession, Charles would probably avoid starting a war with the Ottomans. In OTL, Charles went to war as an ally of Russia against the Ottomans as a consequence of a treaty he had signed in which Russia recognized the Pragmatic Sanction. Without such treaty obligations, Austria avoids the humiliating losses in the Balkans it suffered in OTL. Austria would enter the 1740’s with a less crippling national debt than in OTL. I suspect that Leopold John (as heirs tended to do) would have a different outlook on ruling the Habsburg domains than his father. Unlike in OTL, where Maria Theresa was never giving the training to rule, in TTL, Leopold John would be trained from birth to rule. Leopold John would have been given a seat on his father’s council of state and (as king of both Bohemia of Hungary) practical experience in governing. Leopold John would have been given knowledge of matters of state as well as the abilities of his father’s ministers. Leopold John would also have dealings with the various princes in the Empire, so he would have an idea on how to deal with them once he became emperor.
Assuming Charles still dies from mushrooms in 1740; Leopold John becomes Emperor Leopold II immediately. Leopold begins his reign in a stronger position than Maria Theresa did in OTL. He is emperor as well as king of both Hungary and Bohemia. Leopold does not have to worry about any claims to those thrones and has the benefit of enjoying the loyalty the Habsburgs had built up in those kingdoms. However, Leopold would still have to deal with the various territorial claims that were pressed against Maria Theresa in OTL. For instance, Frederick the Great would still want Silesia in TTL as well as duchies of Julich and Berg. In TTL, Frederick would not invade Silesia without securing the alliance of a country powerful enough to help him keep it. If Leopold is smart in TTL, he would recognize Frederick’s claim to Julich and Berg in order to prevent an alliance between France and Prussia. In TTL, France would build up a coalition with Spain, maybe Sardinia, and possibly Prussia depending on the resolution of the claims to Julich and Berg. I do not see Bavaria and Saxony joining such a coalition unless Austria is already losing in a war. While France was building such a coalition, I see Leopold building a series of defensive alliances with Great Britain, Russia, and possibly the Dutch Republic.
With Austria in a stronger position financially and militarily than in OTL, and with the help of both Great Britain and Russia, Leopold should be able to preserve his domain in TTL. Leopold, with the help of Great Britain, should be able maintain his realm against depredations of the Bourbon states in TTL. If Leopold avoids war with Prussia, there is no reason why he and Frederick can’t be allies in TTL. The two of them could determine the fate of Poland in this scenario.