Stretch'd forth his little arms and smil'd."
Wakefield, in one of his notes, remarks on this—
"An allusion perhaps, to that verse of Virgil,
'Incipe, parve puer, risu cognoscere matrem.'"
Instead of Virgil, I suspect that Gray was thinking of the first Nemean Ode of Pindar, wherein the infant Hercules is described as strangling the snakes sent to destroy him by Juno:
"ὁ δ' ὀρθὸν
μὲν ἄντεινεν κάρα,
πειρᾶτο δὲ πρῶτον μάχας,
δισσαῖσι δοιοὺς αὐχένων
μάρψας ἀφύκτοις χερσὶν ἑαῖς ὄφιας."
Let me give a portion of Cowley's translation:
"The big-limb'd babe in his huge cradle lay,
Too weighty to be rock'd by nurse's hands,
Wrapt in purple swaddling bands;