“every alien pen hath got my use”
'Alien' here means foreign, outside, belonging to another — not a stranger from another country but a rival who has no prior claim on the young man's favor. 'Pen' stands for the poet who wields it, a common metonymy. 'Got my use' means adopted my practice: the other poets have picked up the speaker's habit of drawing inspiration from the youth.
historical The Fair Youth, the Dark Lady, and the Rival Poet- TLN 1083rhetorical device
“taught the dumb on high to sing, And heavy ignorance aloft to fly, Have added feathers to the learned's wing”
The second quatrain builds a contrast: the youth's eyes lift the speechless ('dumb') to sing and the uneducated ('heavy ignorance') to soar, while also giving the already-gifted poet extra lift ('added feathers to the learned's wing'). The flight imagery runs in two directions at once — it raises the low and supercharges the high — making the youth's influence universal.
“that which I compile, Whose influence is thine”
'Compile' in Elizabethan usage meant to compose or put together in writing — not yet narrowed to the modern sense of assembling others' work. The poet is simply saying 'what I write.' 'Influence' retains its astrological sense: a power flowing from the youth into the poem, the way stars were thought to pour invisible force down onto earthly life.