After hearing "Your boy is just like a son to me" for several years they might indeed see nothing suspicious in him buying the boy a car. After all, isn't that what fathers do for their sons?
Well I am a guy and have never been a single mother so obviously my judgment in the mater is suspect.
Small gifts might slide by me (things costing no more than a couple of bucks).
But alarm bells had better go off when some teacher (who is probably unmarried or long divorced) starts giving gifts to a teenage boy that cost hundreds of dollars.
After all the most a teacher makes is what $60,000 maybe $70 if they do a lot of extra curricular activities.