As you said she left college, is she at home during the day with the baby, everyday?
Is she providing good, safe care for him when she is with him?
If she looks after him well, but is craving space I would look in to some college courses that she could potentially pick up now and not just wait for September, which is a long time away really. If you approach your local college, they may be able to work with you to offer her something now. It's not uncommon for mid year starts depending on the course.
Childcare can either come from family or she will be entitled to free childcare until she is 19. The government scheme for this is care to learn.
As another poster suggested, you could organise a set evening or two a week where you will babysit him in the evening.
Providing structured child free time and a way for her to be around other teens like at college may make some positive differences if she is otherwise a good mum and keen to keep and raise her baby.
She definitely needs form boundaries about not leaving him without prior arrangement and a sturdy contraceptive.
Contact the HV for any groups/support aimed at young parents.
If she just isn't interested in her baby or wanting to raise him at all, then there may be some more serious conversations to be had about what the future holds.
I had my daughter when I was 16 (was pregnant at 15). But I had nine months to prepare mentally and was very stubborn about caring for her myself. However, my mum was an amazing support in that she took care of the housework so that my only real 'job' in the house was my baby and encouraged me to continue with GCSEs, 6th form, Uni etc. with my peers. Having child free time even just to attend 6th form really kept me in touch with friends of my own age, even if I couldn't go out in the evenings with them it. I moved out with my daughter when I started university. We're very close and she's a young adult now, she is also incredibly close to her grandma.
Wishing you all the best OP.