Most likely this is an accommodative esotropia: at near fixation the infant must accommodate, and the associated convergence can cause one eye to turn inwards, particularly if the child is hypermetropic; accommodative squint is characteristically more obvious when accommodating or focusing on an object and is often reduced or corrected by appropriate hypermetropic spectacle correction NICE CKS.
At 9 months, this is age-compatible because accommodative esotropia generally develops between 6 months and 5 years, and it may initially be intermittent when the child is concentrating on close objects before becoming constant NICE CKS.
The apparent unilateral nature usually reflects a fixation preference: one eye fixates on the target while the other deviates, rather than necessarily indicating pathology confined to that eye NICE CKS.
Other considerations include pseudo-squint from epicanthic folds, and less commonly reduced vision in one eye, ocular disease, neurodevelopmental disease, or extraocular muscle/cranial nerve pathology NICE CKS.
Primary-care action: assess corneal light reflexes, perform cover testing where feasible, check ocular motility and red reflexes, and refer suspected or confirmed squint to the paediatric eye service; urgent assessment is indicated if there is leukocoria, abnormal red reflex, restricted movements/incomitance, abnormal head posture, or neurological signs NICE CKS.