Hi, I hope you understand that undescended left testis is totally different from absent or a "yet-to-be-located" testis.
In your heading, you mentioned undescended testis whereas in your query' body, you have mentioned that "has no left testis in abdomen and scrotum": so first, this has to be answered along with confirmation of phenotype & genotype.
Also attaching images of your consults & imaging will surely help in reaching a helpful conclusion!
Regarding "any problems will be created in future", if a testis is left in intra-abdominal location, not only risk of possible future virility but also concern of potential malignancy in later life is there.
Next Steps
Please approach your nearest Pediatrician or Pediatric Surgeon, who will guide you in the workup required (after clinical examination). If intra-abdominal testis, hormonal manipulation can be attempted later as he is just 3.5 months old now.
Health Tips
Please check the abdomen, inguinal canal & scrotum for left testis with (in order of increasing accuracy, USG abdomen & scrotum, CT abdomen & pelvis with/without contrast, MRI abdomen & pelvis +/- MR angiogram).....it shouldn't be so a difficult clinical issue!!