Unfortunately whatever has happened, it does seem as though Milo has sustained a serious injury.
We'd worry about shock, internal bleeding and perhaps damage to his spine and/or spinal cord.
Guinea pigs are a prey animal, meaning they try their best to pretend they are fine even when not. This is so they are not a target for predators. For Milo to be laying on his side, we know he is extremely poorly.
He certainly does need veterinary attention and likely needs some anti inflammatories, pain relief and antibiotics.
He may also require supportive care such as fluids and syringe feeding, especially if his appetite is reduced or he is less able to get to his food and eat.
On top of this, we would want to flush his wound through.
If finances are a concern, it may be possible to have a payment plan or pay on credit.
If eligible, you can seek assistance from a pet charity, but many are not open 24/7 I'm afraid.
At home, we are quite limited as he does need prescription medicine.
We can gently clean the wound with salt water and 'nurse' him by offering regular syringe feeds and water.
Try to keep handling to a minimum and stress levels down.