Wiktionary
sweet on
a. (context idiomatic English) enamor with; fond of.
Usage examples of "sweet on".
Robbie thinks Jamie was sweet on Margaret, but whatever the case she begged and pleaded with him to let her go until he gave in.
I always sensed a bond of unspoken sorrow between these two young men, because Edgar had been very sweet on Letitia, and we all knew Alan had been smitten with Madeline, even though she was about to be engaged to Douglas when she disappeared.