8 mo on a twin sized mattress!?

My son won't sleep long his in play pin but will sleep all night on a actual bed. What are your thoughts about babies sleeping on a twin sized  mattress next to your bed? Keep in mind my son is about to be 8 months at 28.5 inches and 23 lbs. he's a big boy and growing rapidly fast, buying a crib would be out the question for medical reasons it would cause more harm then good. Picture for attention 😊