Sadness in Autistic Child

Ask him if he can draw a picture, pattern or shape of whats making him unhappy, then start asking him about what he's drawn and what it is and means, don't lead him or tell him what you think of feel about what he's drawn and definately don't judge. Often children don't have the language or emotional development to say whats troubling them, quite often adults don't either, but they can often draw a representation of what their feeling, even if it looks like a cow pat, they're still telling you something.

Age 12 or 13 is a very change-involved time of life for young people.  Even before Autism is potentially part of the consideration.

It is good that your Son has said that he feels sad (although unable to explain why).  When children and young people share information about big emotions they experience it is important that trusted adults are able to ensure that the young person knows they have been heard and listened to.  Also, that the door for ongoing dialogue remains open - because things can change further over time and there is no embarrassment in that situation.

One thing extra to consider; with Autism under consideration: sometimes, Autistic people (of any age) are not as granular or specific in their awareness of, access to, and articulation of ...which emotions (and their magnitude) may be experienced.

"Sad" might literally mean sad ...or it might be the closest approximation to an unfamiliar / uncomfortable feeling, sensation or emotion - for which, more accurately, the young person has yet to establish their own reference point. 

"Sad" might not mean quite the same thing from one day or circumstance to the next. 

Frequency, magnitude and energy or capacity to cope with the feeling may vary across time (tiredness, or hunger, or thirst, or stress levels, or competing priorities, or illness, or hormonal changes etc. for unrelated reasons to the actual original stressor).

Sometimes self-prompt / self-check-in cards or scales can be helpful in determining and discussing emotions.  How frustrated / how sad / how willing to participate in a group activity / how calm / how much anxiety etc.  They may also be a mechanism for exploring which settings or situations are to the detriment or positive impact to "sad".  It may also be an important technique for helping the young person to realise that how we feel an emotion (or the degree to which we feel an emotion) is not necessarily a permanent state, or that how sad we feel may not always be the same every day. 

Being told the phrase "this too shall pass" is too adult a concept - without their own reference experience - for many young people to adopt (but that does not mean they cannot be supported to learn for themselves; that sometimes we do experience unusual emotions - and that is OK - and their trusted adults are also OK talking about those new emotional experiences).

There is an app called Molehill Mountain which some Autistic people find helps them to learn to navigate their level and frequency of anxiety:

www.autistica.org.uk/.../how-does-the-app-work

Some NHS guidance and signposted resources:

https://www.nhs.uk/mental-health/children-and-young-adults/advice-for-parents/children-depressed-signs/

Sometimes it is tempting to assume we (and the young person themselves) know the most likely origin of "sad".  However, it is also worth taking a pause and then having the "anything else going on?" type of conversation.  Young people, without necessarily being aware of it, can use the understood "sad" associated with one situation as a sort of shorthand / catch-all summary of how they feel - even when one or more other (maybe unrelated) items are their personal "worry beads" too.  The challenge can be - how to initiate the "anything else?" conversation with a young person ... without them feeling it is too much of an intrusion or a grilling. 

Age 12:or 13 is a confusing stage for many young people (Autistic, or not) - but that is OK - because their trusted adults can usually remind themselves os what life felt like when they themselves were once the same age.  The artform is how to open the conversation (and find ways to keep checking-in with one another ...without it becoming an interrogation).

If professional help seems wise to be engaged - that is OK too.

Unfortunately a three year wait for an assessment isn't unusual at the moment, but if you live in England you are entitled to make use of the right to choose option. 

You can speak to your GP about this.  Unfortunately this doesn't apply to the rest of the UK. 

Here's the link to the NAS pages on depression, which may give you some insights.  Wishing you good luck going forward. 

www.autism.org.uk/.../depression