Could Dewey still help the KMT to hold a position on the mainland?
Let's bear in mind that Dewey will not be inaugurated until January 20, 1949. By that time IMO the outcome of the Chinese Civil War was decided. It is possible, as Jay Taylor argues, that "had Chiang pulled out of Manchuria even as late as the spring of 1948, he might have had enough military strength to hold the line at either the Yellow River or the Yangtze, albeit only with large-scale U.S. military and economic aid."
http://www.thegeneralissimo.net/excerpts.htm But he didn't, and by the fall of 1948, well before Dewey would be inaugurated, Chiang was facing disaster in Manchuria. The decisive defeat for the KMT was the Liaoshen Campaign
https://en.wikipedia.org/wiki/Liaoshen_Campaign which ended in November 1948 with the Communists in control of all of Manchuria and with the PLA enjoying numerical superiority for the first time in the war.
After that, events proceeded very rapidly;
"On November 29, 1948, the Chinese Communists' People's Liberation Army (PLA), fresh off a decisive victory in Manchuria, launched the Pingjin Campaign. They captured Zhangjiakou to the northwest on December 24 and Tianjin to the southeast on January 15, 1949. With the defeat of the Nationalists in the Huaihai Campaign further south, Fu Zuoyi and over 200,000 Nationalist defenders were surrounded in Beiping. After weeks of intensive negotiations, Fu agreed on January 22, 1949, to pull his troops out of the city for "reorganization by the PLA." His defection spared the city, its residents and its historical architecture from imminent destruction. On February 3, the PLA marched into Beiping."
https://en.wikipedia.org/wiki/History_of_Beijing
Given that Dewey would not be inaugurated until January 20, he obviously could not have prevented the fall of Beiping (Beijing).
"After the decisive Liaoshen, Huaihai and Pingjin campaigns, the CPC wiped out 144 regular and 29 non-regular KMT divisions, including 1.54 million veteran KMT troops. This effectively smashed the backbone of the KMT army. [42] On 21 April, Communist forces crossed the Yangtze River, and on 23 April they captured the KMT's capital, Nanjing.[27]"
https://en.wikipedia.org/wiki/Chinese_Civil_War
I can't see three months of added US military aid preventing the PLA from crossing the Yangtze (and in the real world not all the aid is going to get there within three months anyway) and saving a disintegrating KMT. The only possible way to save south China for the KMT would IMO be a quick and massive deployment of US troops--something that nobody seriously advocated AFAIK. Even MacArthur said that anyone who favored sending US troops to fight on Chinese soil "should have his head examined."
https://books.google.com/books?id=DUg2KGMQWHQC&pg=PA396