The regent for Philip will be empress Isabella who is mostly focused on maintaining Spain at this point. Austria is of little relevance to her and I can see Ferdinand being given the hapsburgs lands. The spaniards will focus on iberia at this point and with Charles dead the people will want a king of Aragon and Castile, not a austrian spending money and manpower in foreign lands with no benefit to them.
One might argue, that Ferdinand was more Spanish than Austrian, since he was raised in Spain until he was IIRC 15 (might be 14, depends on the month), whereas Charles had been raised in the Burgundian Lands.
Ferdinand already had the Austrian Hereditary Lands, and he also was king of Bohemia and Hungary (and Croatia), the only Habsburg lands that could be given to Ferdinand was the Burgundian Inheritance.
In this case I can honestly see Ferdinand making a grab for Burgundy. In exchange for respecting his toddler nephews right to spain and its territories in a agreement with regent Isabella. That will most likely butterfly away the trouble that came for spain with the netherlands. Win-win.
I tend to agree, negotiations between Charles and Ferdinand had been very tough IOTL, now with his elder brother gone and his nephew being underage, Ferdinand as the only adult male member of the house of Habsburg has become the head of the dynasty. Not to mention Ferdinand's connections to Spain, he was named after his grandfather , Catholic Monarch, Ferdinand of Aragon.
After the division of their inheritance Charles had kept the Crown of Castille (with the colonies), the Crown of Aragon (with the Italian possessions) and the Burgundian Inheritance. So I agree, that Ferdinand could make a move to improve on the division he disliked, and this would give him an excellent opportunity. Isabella and her Spanish advisers probably would have less (not none) objections than OTL and Ferdinand could directly or indirectly (others might act in his name), be a real potential threat for Philip.
Whereas for Ferdinand, the Burgundian Inheritance would greatly strengthen his position in the Empire, give him more resources against the Ottomans and thus it will make him stronger. Even IOTL Austria-Burgundy and being the Habsburg candidate for the Imperial Throne, is what he had hoped for, since by the point the OTL division had become an issue, the death of Maximilian in 1519, Charles was already king of Castille and Aragon (etc.) and duke of Burgundy etc. The position of Holy Roman Emperor was elective and Maximilian hadn't managed to get one of his grandsons elected King of the Romans before his death. So directly on the table where the Austrian Hereditary Lands and the Habsburg candidacy for the Imperial throne, add to this, that Ferdinand initially wanted to divide everything, so not only Austria and Burgundy, but also Castille and Aragon with his brother. So Austria-Burgundy and potentially the HRE seem to have been his goal, since his brother was already king of Castille, Aragon, etc. since 1516.
ITTL I still see the OTL marriage between the cousins Maximilian II and Mary of Spain happen, though perhaps now to also seal the deal of this new ATL division.
Also note that the duchy of Milan by this point was not yet a Habsburg possession.