I found this info online. Sounds like he might have a lateral lisp which might help you get a quicker referral?
Four types of lisp
- Interdental (frontal) lisp
In an interdental lisp (or frontal lisp) the tongue protrudes between the front teeth and the air-flow is directed forwards. The /s/ and /z/ sound like 'th'.
Children developing speech along typical lines may have interdental lisps until they are about 4½ - after which they disappear. If they don't 'disappear' an SLP assessment is indicated.
Interdental /s/
Words such as 'soup', 'missing' and 'pass', which all contain the voiceless alveolar fricative consonant /s/ are pronounced 'thoop', 'mithing' and 'path'. The voiceless 'th' sound that occurs in a word like 'thing' (or a sound very much like it) replaces the /s/.
Interdental /z/
Words like 'zoo', 'easy' and 'buzz' which all contain the voiced alveolar fricative consonant /z/ are pronounced 'thoo', 'eethee' and 'buth'. The voiced 'th' sound that occurs in a word like 'them' (or a sound very much like it) replaces the /z/.
- 'Dentalised lisp'
'Dentalised lisp' is not an 'official' diagnostic term. It is an expression (like 'dentalised production') that SLPs use to describe the way an individual is producing certain sounds. The tongue rests on, or pushes against, the front teeth, the air-flow is directed forwards, producing a slightly muffled sound.
Typically developing children (in terms of their speech) may produce dentalised variants of /s/ and /z/ until around 4½ years - and then grow out of it. If they don't grow out of it, an SLP assessment is indicated.
- Lateral lisp
Lateral lisps are not found in typical speech development. The tongue position for a lateral lisp is very close to the normal position for /l/ and the sound is made with the air-flow directed over the sides of the tongue. Because of the way it sounds, this sort of lisp is sometimes referred to as a 'slushy ess' or a 'slushy lisp'. A lateral lisp often sounds 'wet' or 'spitty'.
Unlike interdental and dentalised lisps, lateral lisps are not characteristic of normal development. An SLP assessment is indicated for anyone with a lateral lisp.
- Palatal lisp
Palatal lisps are not found in typical speech development. Here, the mid-section of the tongue comes in contact with the soft palate, quite far back. If you try to produce a /ç/ - or a 'h' closely followed by a 'y', and prolong it, you more or less have the sound.
Unlike interdental and dentalised lisps, palatal lisps are not characteristic of normal development. An SLP assessment is indicated for anyone with a palatal lisp.
'sh', 'zh', 'ch', 'j'
Sometimes children and adults who lisp when they attempt to say /s/ and /z/ also have tongue placement problems when they try to say 'sh' as in 'shoe', 'zh' as in measure, 'ch' as in 'chair', and 'j' as in 'jump'.
They may dentalise these sounds, or produce them interdentally, or produce them with lateral air escape, or with excessive palate to tongue contact.