Odd red marks appearing on my son

hopefully someone can help me , lately odd red marks have been appearing on my 10 year old sons arms and on on his hand between his finger and thumb .
he hasnt hurt himself and the marks look like a small red bruise some times two at once about the size of a 20 cent piece . they fade after a couple of days.

the first two appeared above and below his elbow on the bicep and fore arm and then a few days later on the same arm higher up .

he is not running a fever and they are not sore .
another one came this morning on his hand between his finger and thumb.

he has no allergies to food and is not on any meds . he pretty much a health lad .
they dont appear after any particular activity , just totally random.

any advice is appreciated .