I see your second picture... yup, looks like a swollen abdomen. Don't put him with Mama, if you haven't. It would be uncomfortable to have the pressure of her abdomen. 100 F is all right to keep him alive on your porch, but I would give him another source in his little nest, for his own peace of chick-mind. From my experience, the little chicks love the infrared, even when it is 100F, and they like red light. Could you put a table lamp near him with a bright bulb and a red cloth (bandana?) hanging between the lamp and the puffy chick?
That's if you don;t have a heat lamp. Or some Christmas lights that are red, hung near him, would help, too. Red light is a bit calming. It's like what they see inside the shell (?0 or beneath Mama's feathers (?)