Nail biting and finger nail infections

Hi, My son is almost 4 years old and on the waiting list for an autism assessment. He has a strong oral fixation and continues to bite his fingers. We are struggling to get him to stop biting his fingers, he has a speech delay and doesn't completely understand the implications of him biting his fingers is causing infections in his finger nails. To combat the infections we've been perscribed oral antibiotics which he is refusing to take, so doesn't help the situation as his infections are not improving due to him not taking his medication.

To try and take his antibiotics we've tried just giving them to him and telling him but he refuses, placing into his favourite drinks but generally tastes it and stops drinking. Placing into nice foods again will taste and stop before taking an entire dose. I was hoping to see if anyone has had any related experience of children refusing to take antibiotics and how they got them to take them. And also any experience in children that bite their fingers to the point of causing finger nail infections and what they have done to help stop them from doing this. He will not keep on gloves. I have just purchased some nasty tasting nail vanish to see if that will stop him from biting his fingers. When trying to stop him from placing his fingers in his mouth it usually ends up in him becoming triggered and then causing challenging behaviours to occur. 

Any advice would be appreciated as we are struggling with how to solve this issue. 

Thank you.