Does anyone have any tips on how I could get him to go down in this thing?! I'd be so grateful
Yes 
Feed and Wind Well
You will never get a baby who is hungry to sleep, certainly not sleep independantly (without you). If in doubt feed, feed and feed some more.
Adequate winding is also very important. After every feed try to get at least one burp from baby. Try reoffering milk after winding, then wind again. If you cany get baby to burp, try different winding positions.
Swaddle
Sleepyhead is great but is expensive. A swaddle taps into the same feelings for a fraction of the cost. A flat cot sheet or a giant muslin square cost only a few pounds and are easy swaddled.
This is about recreating womb-like conditions: tight, enclosed, restricted, protected. Agitated networks are often calmed once settled in a swaddle. It also reduces stimulation to help out baby down.
Dummy
Teamed with the swaddle works best. The swaddle calms flaying agitated limbs and the dummy allows for comfort sucking, which babies find naturally soothing and calming.
A dummy is best use at this age once baby is calm, to go from awake/calm to asleep. The dummy is rarely any use in a newborn when put into a screaming mouth. So calm baby first, see above and below.
Rhythmic Movement
Swishing and swaying often works best, since it recreates the movement baby felt in the womb. Also works best alongside swaddle and dummy.
The idea would be to take a fractious baby (who already has a full tummy) put in swaddle, quite tight against the shoulders, free by the hips. Depending how agitated baby Is, this may calm but if very agitated it won't.
So once swaddled, try the 'on the shoulder' position while you sway, this allows for winding at the same time. If not a cradle hold and some people try a face downwards cradle hold. Gentle, rhythmic swaying from side to side. As baby start calming, give dummy to calm further into sleep.