My son had minor letter pronunciation issues at the same age. He saw a speech pathologist and we were told it’s nothing to stress about. He learnt to say all letters properly with time

In my head all I can think is to keep singing la la la la la and see if that works?

I had a friend with a similar speech problem and she organised a few visits with a a speech therapist and after that three week period of three lessons all was well.

Hi, the sound ‘L’ is a later developing sound and the error sound (‘Y’) is a normal developmental error, and age-appropriate for a 5 year old, even though many children will have developed the ‘L’ sound by this age.
‘L’ is made with your tongue tip placed behind your top teeth, in a similar position to placement for ‘T’. The first thing is to have him recognise when sounds are the same or different- you can do this by hiding your mouth with your hand and saying Y then L, L then L, L then Y, randomly until your son is identifying same and not same consistently. If this is too hard, go back a step and try to have him identify other things that are the same or different- socks, pegs, plates, cutlery, etc. You can look up information on Caroline Bowen’s website- she is an Australian Speech Pathologist well respected in the field.
Once your son can recognise the L as distinct from the Y, then you can see if he can copy your mouth as you make the sound- a mirror can help him see where the tongue goes. Y is a sound made with a hollowed tongue shape at the back of the mouth, so if you can get the forward tongue position, sometimes that is enough. You can start a scrap book with pictures of L words such as lemon, lace, etc, and another page of Y words such as yoyo, yak, yellow.
As you share books, you can point out things that start with ‘L’ and have him tell you if they are being said correctly or not (make mistakes sometimes with the ‘y’ sound so he gets practice hearing you make his error sound as well).
As he has developed the other sounds , I feel it is likely he will develop this sound as he gets closer to six, but these are some ideas that might help nudge him along. Best wishes,
Dianne

Have you considered having her assessed by a speech therapist?

If you speak to your GP you can get a referral for some free speech pathologist lessons. Also, you can try and tell them to show you the action that they are pretending to lick a lollipop for the ‘l’
sound.

It would be worth seeing your doctor and getting a referral to a speech therapist. I had therapy as I had trouble with my s sounds at the same age. A speech therapist will do some sessions with your child and give you exercises to practice at home. They’ll try and make it as fun as possible to engage your child as well.

You might find this a helpful read
https://mommyspeechtherapy.com/?p=69

Also, practice in front of the mirror

My son (6) has the same issue! I show him and explain to him where he needs to put his tongue (behind the top front teeth) but he will keep putting it between his teeth. I brought it up with his speech therapist and she said it’s something he will need to work on. Right now we are focusing on R and Sh sounds. He used to get them so easily but not anymore. Maybe try for a speech therapist if you think your little one needs help